CLASSICAL Alvin Lucier: ''I Am Sitting in a Room.'' (Lovely Music/Vital records, VR 1013.) Alvin Lucier likes to set a process in motion, then follow it to its logical conclusions. Here he speaks a few sentences into a tape recorder, then re-records them with a second machine, then re-records again . . . and so on, for 32 ''generations,'' spliced together for this disc. The beginning of the record sounds, naturally, like regular talking. By the midpoint, the sound has started a transformation into something rich and strange, verging on music. By the end, the miracle is complete: Speech has disintegrated before our very ears, words and meaning lost in a shimmering mist of bell-like ''musique concrete.'' It's one part recital, one part physics lesson, and one part magic, concocted by an offbeat but most inventive composer.