I think the Chaos Emerald and Special Stage 1UP jingles were left out. And ofcourse, songs that were re-used for Sonic 2 were only included once. Drowning was also left out (who doesn't hate that track anyway?).
...... And if I'm not wrong, the last boss theme from Sonic 2 was actually... left out. This would be the most major fuck-up in history. I hope they did a screw-up on making the tracklist or something.
EDIT: I guess "Death Egg Zone (Part 2)" could be it
