I hate to bump a semi-old topic, but apparently that solution isn't quite good enough. I can't think of a good way to describe it without, well, completely describing it, but I guess I have to in order to find a proper solution.
What I want to do is replicate what happens with Sonic's Speed Shoes. When you pick them up, the music gets faster without changing pitch, and when time runs out, the music reverts to its normal speed. My previous method covers everything EXCEPT when times runs out -- if you switch back to the normal track, it won't be matched together since the other one was sped up.
Basically, the ideal solution would speed up a single track, without changing pitch whatsoever. If there's a method of changing pitch, that'll work too, since I can change the frequency ratio (which makes it higher pitched) and then correct the pitch separately afterward. If I can't, I'll have to settle for the method most Sonic fan-games stick with: use a completely different song for Speed Shoes.
Is there any way to do any of this?