Not since Keith Partridge belted out ''Come On, Get Happy'' have music and the sitcom meshed as well as they do on The Adventures of Pete & Pete. The show, which features some of the best alternative music around, chronicles the strange lives of brothers Pete, 15 (Michael Maronna), and Pete, 11 (Danny Tamberelli). Dad owns a driving range, Mom has a metal plate in her head that picks up radio waves, and strongman Artie is always around to rescue Little Pete. Since the show grew to half-hour specials in '93, it's been one of the network's most popular, helped by cameos by the likes of R.E.M.'s Michael Stipe and tracks by such groups as Ministry and Poi Dog Pondering. It's now on weekly one can only hope success doesn't go to Mom's head, alongside the plate.