I'm pretty sure as far as the tracks that are in SRB2 go, the originals are included too. Just go to options and turn off digital music, then use the following commands
TUNES 91 - Nimbus Ruins Zone / Empyrean's Channel
TUNES 92 - Jade Valley Zone / One of the Unknown tracks, I forget which one
TUNES 93 - Frost Columns Zone / Cave
TUNES 95 - Desolate Twilight Zone / Another Unknown track
TUNES 98 - Airborne Temple Zone (1.08) / I'm not sure if this was a S:EQ track or not. I can't remember where it came from.
Also, Midnight Freeze Zone from Mystic Realm uses the Ice Forest music from S:EQ, though it was remixed in 2.0. Thankfully, I have the original. It's much more calm and relaxing than the newer one.
http://www.mediafire.com/?g4j5cul4zcl4bgv
Additionally, other S:EQ music was used in levels packs a while back. I know Tets used "Underwater" for Dark Palisades Zone, and I used "Eggman Boss Battle" for Slime Silo Zone. There are probably more, but I don't really remember.