ROCK/POP Sammy Hagar: ''VOA.'' (The David Geffen Company GHS 24043.) - The title track , ''VOA'' (Voice of America), represents the core of this album's theme. Bring today's young people together, and show them the importance of national unity - this is Sammy Hagar's message. Written in one month and recorded in two weeks, ''VOA'' reflects the spontaneity with which it was produced. ''Burning Down the City'' is a song about graffiti artists who have their work painted over, lawbreakers and the way they talk. In sharp contrast, ''Swept Away'' rocks back and forth on the ocean waves that it portrays. First learning his field with the band Montrose, Hagar has been one of the few musicians to break away from his band and succeed. Being a fairly adept songwriter has helped him survive. Assuming he remains prolific and proficient, I expect to see him in his music scene for some time.