It is "Yeah!" by GRG, transformed into my own player due to rastertime problems. I think you can consider the version in HVSC the *real* one, eventhough the one in the demo sounds better ;).
It's a shame that all the songs just say "player by HCL". My intention always was to change that before the demo was released, but of course i screwed it up :(. 3 tunes by Agemixer (Burp, Dark Space, Audio man (in-game tune 2)) have the same properties, and "Wet Jazz" by Mindflow in the slime-pic part.