Maryland Cup Corp., the nation's largest manufacturer of paper and plastic food-service products, said earnings rose 16 percent in the year ended Sept. 30 to a record $30.5 million ($4.52 a share) from $26.1 million ($3.89) a year earlier. Sales jumped 12 percent to $647 million, also a record.

The fourth quarter was the best for the 71-year-old firm, which is based in Owings Mills, with profits of $13.4 million ($1.98) compared with $12.9 million ($1.91), and sales rose to $194 million from $175 million.

Two of the major electric utilities serving the Washington area reported slightly lower profits for the 10 months ending Oct. 31.

Potomac Electric Power Co. said net income totaled $95.6 million ($1.88 a share) compared with $95 million ($1.96) in the same period last year.

Revenues totaled $856.7 million compared with $729.1 million last year.

For the 12 months ended Oct. 31, Pepco had income of $194.6 million ($2.03) compared with $196 million ($2.16) for the comparable period a year ago. Operating revenue for the 12-month period was $983 million versus $839 million.

Virginia Electric & Power Co. said earnings for the first 10 months fell from $190.5 million ($1.51 a share) last year to $185.39 million ($1.36).

Net income for the 12 months ending Oct. 31 was $236.46 million ($1.77) compared with $224.25 million ($1.77) in the comparable period a year ago.

Waxie Maxie Quality Music Inc., a Washington-based music store chain, rebounded from a loss last year as sales increased more than 20 percent in the fiscal year that ended July 31.

Net income for the latest fiscal year totaled $75,400 (37 cents a share) compared with a loss of $6,200 (three cents) in fiscal year 1980. Sales increased 20.7 percent to $1.09 million. The company doesn't release separate figures for each quarter.

Washington Homes reported a first-quarter loss of $273,078 compared with a profit of $8,552 (a penny a share) in last year's first period. Sales fell sharply from $7.2 million to $3.1 million.

USAir reported net income in October fell 41 percent to $5.67 million (33 cents a share) from $9.6 million (77 cents) in the same month last year. Revenues increased 6 percent, from $93.9 million in October 1980 to $99 million.

Net income in the first 10 months showed a sharp decline also, going from $51.7 million ($4.06) in 1980 to $44.1 million ($2.74).