IF YOU wondered just why Richard Dreyfuss was in such agony in "The Competition," then you will be interested to know that there is over a half-million dollars out there waiting to be collected by the pianists, violinists, cellists, singers and composers who take the trouble to enter -- and have what it takes to win -- some of the competitions coming up in this country in the next 18 months.

And the professional and emotional complications that grip the closed little world of that San Francisco contest in "The Competition" are anything but at odds with reality.

These competitions occupy a controversial spot in our music world today. They are often marred by such charges as that:

Some judge was at some time a teacher of one of the contestants;

The judges "completely ignored the clear mandate of the audience and awarded the grand prize on some wholly quixotic ground."

Mothers of contestants have invaded the secrecy of judges' deliberations with threats of dire consequences if their children were not named the top winners.

And once a famous judge told his fellow jurors that it was "obvious that Pianist X was light years ahead of every other contestant," whether or not they realized it. It subsequently came to light that the judge was sleeping with the young competitor, a condition that is now specifically forbidden in the rules laid down in that competition.

The Soviet Union has used the international competition circuit as a carrot-and-stick for its leading young talents for years. Official committees decide which young artists will enter what competitions, as with the lonely Russian girl in "The Competition." And the rankings attained by their contestants determine where and how well they will live in the future and just how their artistic careers will be permitted to continue.

Dramatic tensions surpassing anything in "The Competition" have surfaced in the course of various contests.

Just before the first Cliburn Competition opened in Fort Worth, Tex., in 1962, its founder, the late Grace Lankford, received a number of threatening letters, one of which, though anonymous, said, "If a Russian wins this contest, you are dead!" As it happened, Ralph Votapek from the United States won, removing that particular threat.

However, Fort Worth has had its share of competition fireworks. In 1977, Steven De Groote was named grand-prize winner, a decision which so enraged some of the wealthy partisans of the U.S.S.R.'s immensely talented Youri Egorov that they promptly got together and awarded him a special prize of $10,000, the same amount given to De Groote, so that Egorov could also enjoy a New York City Debut. At the moment, it appears that the Egorov career is enjoying greater acclaim than De Groote's. a

One of the constants in any competition situation is the fact that there are some performers who do not function at their best in the midst of those kinds of turbulent trials. There was a remarkable unanimity of opinion among the judges at one major contest that the most poetic, and very possibly the most gifted of all the entrants, was a young Russian who was obviously ill at ease in the face of the rigorous demands. He ended up with fourth prize, though a majority of the judges felt that he was the pianist they would most like to hear in concert. It had also turned out, in the course of the two weeks required for the contest, that this pianists age had been officially falsified on his entrance application. He was actually four years over the age limit. A phone call to the Soviet desk of the State Department asking for advice brought the response, "Do nothing unless he wins. In that case, call us back." Another crisis averted.

Many of the major competitions award both cash prizes and guarantee concert appearances whose value is inestimable. One of the newest of these is the Pavarotti Auditions under the auspices of the Opera Company of Philadelphia. These finals are to be held in Philadelphia in May, and no cash prizes have been mentioned at all. But since the winners will appear opposite the superstar tenor a year later in three performances of "La Boheme" and two of "L'Elisir d'Amore," they are automatically assured of a brilliant launching into a career.

No career is guaranteed the winners of the Metropolitan Opera's annual regional auditions, but as many of their finalists have gone on to major appearances as have been absorbed into the routine of small roles in the big company.

It has been interesting to note that judges of various contests have, in recent years, refused to give any first prize, pointing out that the large rewards demand equally high standards of performance. The Leventritt Grand Prize was not given for 10 years, during which decade the judges consistently ruled that none of the contestants were good enough. The judges in last summer's Maryland University contest made a similar decision, giving two second prizes but no first.

Composers continue to get the short end of the stick when it comes to the cash awards. Out of more than a quarter of a million dollars that will be awarded between now and next September, approximately $12,000 is earmarked for those who write music. Bach and Mozart would not be at all surprised, though Verdi and Richard Strauss would. Pianists will come in for the lion's share this year, with something like $190,000 waiting for them in Fort Worth, New York City, Cincinnati and other cities. This outsized sum will shift locales from year to year, however, depending on the Kennedy Center-Rockefeller Foundation Competition for Excellence in the Performance of American Music, a contest that rotates annually between pianists, violinists and singers.

But singers will not do badly this year, thanks to the Montreal Competition, whose prizes total around $22,000. Without adding in the Met auditions, vocal contests will shortly be handing out $32,700 in cash.

If we take a look ahead, which is always advisable in view of deadlines for entering the big contests, a huge new entrant has just surfaced: the International Violin Competition of Indianapolis has been announced for September 1982. Its prizes of $50,000 put it in the very top cash category of world contests. Subtract that amount however, and the prize money for violinists and cellists dwindles to a paltry $10,000 plus, except of course in the year when the Kennedy Center-Rockefeller Foundation trials are open to violinists.

With those figures dangling in front of them, it is no wonder that young artists who have worked for years, practiced interminable hours and learned the dozens of compositions required for the various competitions, die a thousand times during the final, hectic weeks and days while judges make up their minds. Insomnia if rife during the finals, while some contestants throw up every time they have to walk out on that stage and play one more time for the judges. But the lure is, for most, irresistible. Here are dates, deadlines and prizes available in some of the contests coming along in the immediate future: FOR PIANISTS

The Rockefeller Foundation-Carnegie Hall (just moved from the Kennedy Center) Competition for Excellence in the Performance of American Music, no age limit. Deadline, Feb. 17.Prizes worth $96,500. Write Susan Clines, Carnegie Hall, 881 Seventh Ave., New York, N.Y. 10019.

Dealey Awards, ages 17-28. Prizes totaling $2,750. Contest March 12-15. Write Dealey auditions office, P.O. Box 2977, Dallas Tex. 75221. Same address and dates apply to the Dealey Awards for violin, cello and voice; singers' age limits are 20-30.

Friday Morning Music Club (of Washington.) Deadline just passed. Ages 18-28. Prizes round $6,000. Call Jane Lea, 202-966-8811.

American Music Scholarship Association. Deadline: March 4. Prizes around $15,000. Ages 5-25. Write: AMSA, 1826 Carew Tower, Cincinnati, Ohio 45202.

Young Musicians Foundation. Deadline: April 1. Ages: under 21. Prizes: $3,500. Write Sylvia Kunin, YMF Debut Competition, 914 S. Robertson Blvd., Los Angeles, Calif. 90035. Same address, ages, and prizes for violinists and cellists.

Boyd Competition (Augusta Symphony). Deadline: April 13. Ages 16-26. Prizes: $2,000. Write: Augusta Symphony, 619 Bourne Place, Augusta, Ga. 30904.

Cliburn Competition: Deadline is past, but you might want to go to the contest May 17-31. Prizes: $36,500. Write: Cliburn Foundation, 3505 West Lancaster, Fort Worth, Tex. 76107.

Maryland U. International Competition. Deadline: April 1. Ages: 18-30. Prizes: $10,000. Write: Fernando Laires, Md. U. International Piano Competition, College Park, Md. 20742.

Beethoven Society (Washington).Contest April 11. Ages 12-18. Prizes $1,750. Write: Mrs. Clarence Fisher, 5000 Klingle St. NW, Washington, D.C. 20016. FOR SINGERS

Metropolitan Opera Regional Auditions (in Washington): March 5-7 For deadline and details, write Mrs. William Ragan, 10020 Chapel Road, Potomac, Md. 20854. Winner could go far.

Montreal International Competition. Deadline March 1. Ages 20-35. Prizes $22,000. Write: Montreal International Competition, 106 Dulwich Avenue, St. Lambert, P. Q. Canada, J4P 2Y7.

New Jersey State Opera Scholarship Auditions. Deadline: March 3. Ages 22-34. Prizes $5,500. Write: Barbara Cromarty, Auditions Dept., N.J. State Opera, 1020 Broad St., Newark, N.J. 07102. FOR VIOLINISTS

Naumburg Foundation Internation Competition. Deadline March 1. Ages 18-32. Prizes $3,000. Write: Naumburg Foundation, 144 West 66th St., New York, N.Y. 10023.

International Violin Competition of Indianapolis. Deadline March 31, 1982. Ages 18-30. Prizes $50,000. Write: Thomas Beczkiewicz, Director, International Violin Competition of Indianapolis, 320 N. Meridian, Suite 511, Indianapolis, Ind. 46204. FOR COMPOSERS

Kennedy Center-Friedheim Awards. Deadline July 15. No age limit. Prizes $7,500. Write: Pat Salisbury, Kennedy Center Friedheim Awards, John F. Kennedy Center, Washington, D.C. 20566.

Politis Competition (Boston University). Deadline Jan. 31, but you might try. No age limit. Prize $3,000. Write: Politis Competition Prize, Boston U. School of Music, 855 Commonwealth Ave., Boston, Mass. 02215.

Annapolis Fine Arts Foundation Competition. Deadline March 1. Age, under 30. Prizes $300. Write: Annapolis Fine Arts Foundation Inc., P.O. Box 228, Annapolis, Md. 21404.

These are by no means all the competitions now in the works.Others, on which details are at the moment missing, would include the Three Rivers Piano Competition, sponsored by the Pittsburgh Symphony, the Baltimore Opera Auditions, one of the largest in the country, and the J. S. Bach International Piano Competition, whose next contest will be held in Washington late in 1982. For details on these, write to the appropriate organization. For the Bach contest, write: Raissa Tselentis, 1211 Potomac St. NW, Washington, D.C. 20007.

Perhaps you should go see "The Competition" again.