Videogame music is probably your most direct resource, but obviously it depends on what you need it for. I mean, if your looking to sample instruments your probably better off with normal tunes and just taking as little as you can from them to make your own sounds.
A good place to hunt for free music that you could use in your game would be Modplug, like root around for soundtracker modules that you like then contact the composer for permission - most of the time they'll glad to let you use their tune in a game. Modplug tracker is quite good, it can create .wav's from the output which you'd convert to .mp3 for ease of use.
You should also be aware that .midi tunes suck unless you have great sound hardware, and they rarely have any singing
(midi tunes are digital, they can't really hold recorded sounds).
Van-B
