I'd check these first thing:
1.) If you are using one program to play your music, try another - maybe it's just a program setting that got accidentally changed. After all, most programs, currently, seem to have some sort of "hot keys" built in by default.
2.) Check your Audio settings under Control Panel, to make sure no weird changes are made there.
3.) Though highly unlikely, I'd try several virus/spyware checks if the others don't pan out. I mean, it'd be possible for someone to use malware of sorts to speed up your music, but I just can't see anyone actually wanting to do that. I've been surprised by stupidity before, so if that's the case, it won't be the first surprise.
If I had to make a guess, regardless of anything, I'd say it is a software-specific setting. I know that you can speed up and slow down music in just about any major program that will play it, nowadays.
Try that and let us know.