Most people understand the VS Fang track in SRB2 as being a remix of his theme from Triple Trouble, and things like the Sapphire Falls multiplayer level remixing Green Hill's ever-pervasive tune. Of course some music even originates from other properties outside of just Sonic games (NiGHTS, Mario, the Egg Rock disco room/Disco Zombie music as coming ripped from Earthworm Jim 3D, Zero Ring referencing Zero Wing, and the previous Sonic Team Jr. intro with the Pure Fat truck - that tune actually derives from ClayFighter 63⅓'s title screen music!). What's most interesting to me though when it comes to references is Jade Valley's music: it's a light remaster of a MIDI from what was left of the Sonic: Emerald Quest fangame, sort of faintly relating to SRB2 which more about it can be read
here, and either "Tails' House" or "Untitled 3" is what the MIDI file is called. In turn, it recycles a portion of the music played during the first two acts of Sunset Park in Triple Trouble, welcomely not the more popular Act 3 Marve Shupopolus-Gou train boss theme!