Complex sentence: While practicing the song outside the sound booth Al ran into John "Bermuda" Schwartz who told Al he was a drummer and agreed to bang on Al's accordion case to keep a good steady beat to the song.
Split into simpler sentences: While practicing the song outside the sound booth, Yankovic ran into Jon "Bermuda" Schwartz who told Yankovic he was a drummer. Schwartz agreed to bang on Yankovic's accordion case to keep a good steady beat to the song.