Even in the early days,when he had precious little else. Berry Gordy Jr. had plenty of nerve. Others may have seen his Motown Records for the bedgling it was then, but to Gordy it was " The Sound of America." To the outside world the firm's headquaters may have looked like a run-down house on Detroit's West Grand Boulevard; to Gordy it was no less than " Hitsville U.S.A." He slept on the second floor and produced records downstairs during the day. This year, as it turns 25, Mowtown Industries is the largest black-owned company in America ( with revenues in 1982 of $ 104 million ), and Berry gordy doesn't sleep over the store anymore. Now he lives in a hilltop estate, and Mowtown's offices fill three floors of a Hollywood high-rise, where 200 employees oversee an entertainment empire that includes a music-publishing house and a TV/movie production arm. But the heart of it all is still Mowtown Records, with superstars Stevie Wonder and Lionel Richie heading the play-list, punk/funkster Rick James in the second rank and new acts like DeBarge just starting to break big. Five of the top 30 on Billboard's black singes chart last week were Mowtown releases says an old competitor. Alantic Records chairman Abmet Ertegun, they make the records they know how to make, and they still make them better than anybody else.