On Thursday night Horace Tarbox sat in an aisle seat in the fifth row and witnessed "Home James." Oddly enough he found that he was enjoying himself. The cynical students near him were annoyed at his audible appreciation of time-honored jokes in the Hammerstein tradition. But Horace was waiting with anxiety for Marcia Meadow singing her song about a Jazz-bound Blundering Blimp. When she did appear, radiant under a floppity flower-faced hat, a warm glow settled over him, and when the song was over he did not join in the storm of applause.