Few musicians can boast the range of Dave Koz: He's played sax with Arsenio Hall's posse and has appeared twice on General Hospital. Last year, Koz, 31, and his brother, Jeff, updated the soap's theme. ''I thought, if 10 million fans hate this, they'll hate me and my career is over,'' Koz says, laughing. In fact, ''Faces of the Heart'' proved so popular that Koz released it as the latest single off his Lucky Man CD. A big soap fan since childhood, he got Emma Samms (who played Holly on GH, intermittently from 1982 to 1993) to join him in the video. Now on tour, Koz still tries to keep up with GH. ''It doesn't matter how many times I hear it,'' he says, ''when I hear the theme I think, 'This is sooo cool.' ''