The first Pulitzer Prizes in music were awarded in 1943, when William Schuman won for his "A Free Song." But just suppose there had been a music Pulitzer in 1893 - who would have won it?

The answer has to be: Horatio Parker. That was the year his oratorio, "Hora Novissima," was published and performed, winning him fame that quickly spread from Boston, New Haven and New York to England, where the new work became the first American composition to be performed at one of the famous Three Choir Festivals.

Edward MacDowell would have been Parker's only serious competition, and since his Second Piano Concerto would have won the prize several years before, he would not have been in contention. As for Charles Ives, he was only 19, and his "awful" music was not going to cop a Pulitzer for another 54 years.

These prize thoughts come to mind today because of the program that Paul Callaway will conduct this afternoon at 4 in the Washington Cathedral.The Coral Society and the New Century Singers will join in singing music by no less than four winners of the esteemed prize: Ned Rorem, who won the award in 1976 for his "Air Music"; Leo Sowerby, whose "Canticle of the Sun" won in 1946; Charles Ives, who was belatedly awarded the prize in 1947 for this Third Symphony, and John LaMontaine, whose Piano Concerto, given its world premiere by the Natinal Symphony under Howard Mitchell with Jorge Bolet as soloist, won the prize in 1959.

Rorem will be heard today in his "Missa Brevis," with Charles Olaker, Beverley Benso, Dougllas Robinson, and Richard Dirksen as soloists. Sowerby will be represented in his setting of "And They Drew Nigh"; Ives' evocative music for Psalm 90, with organ, soloists and bells in on the program.

And there will be the world premiere of a new work by LaMontaine, who has turned, for the new composition, to poetry by John Greenleaf Whittier. Much of the material has been used previously for hymns, some of it having been long popular in a variety of hymnals. The most famous lines to be heard today are the verses beginning, "Dear Lord and Father of Mankind," which come from a long poem strangely entitled "The Making of Mead." Strange, that is, when you think how firmly entrenched its mushy musical setting by Frederick C. Maker has long been in hymnal after hymnal.

LaMontaine has sought, in his new music, to give Whittier's lines appropriate strength and character in keeping with their basic philosophy.Writing the accompaniments for organ and guitar, he hopes the new music will be attractive to both large choral groups such as today's, and to smaller choirs.

Three non-Pulitzer composers also will be heard today. Horaito Parker, who was for years the head of the school of music at Yale, is on the program with a work of unabashed romantic flavor, in which he captures with deep insights, a genuinely magical atmosphere. It is a setting of the Latin poem, "Iam sol recedit igneus " ("Now Sinks the Golden Sun to Rest").

With descending vocal lines, Parker's music is a radiant mirror of the text.

The day's othere two composers are both interesting possibilities if you care to conjecture about future Pulitzer winners. Lee Hoiby's operas, of which "Natalia Petrovna" was handsomely mounted by the Opera Society of Washington under Paul Callaway's baton, and whose "Summer and Smoke" could well have won the Pulitzer the year it was produced, may at any time make him a winner. He will be heard in a setting of "Hear Us, O Lord."

John Corigliano works easily in the larger classic forms of the concerto and sonata, as well as in song cycles. There have been years in which it would have been no surprise to see his name on the select Pulitzer list. His music today is a version of Psalm 8.

One of the things that makes this afternoon's program in Washington Cathedral of special interest is the gathering together of diverse works covering three-quarters of a century in American composition. Its caliber is such that no other country could surpass these works in the same genre. CAPTION: Picture, Composer Horatio Parker: He would have won.