Audio pitch problems

I'm not sure what started to cause this or why, but I'm having a few problems regarding the playback of certain sound files and midis on my computer.

The problem goes like this: whenever I play back a midi or a Windows scheme sound, the pitch is slightly lower than it should be. I don't know what to do with the Windows sounds, but I can fix the midi problem by making Winamp's midi output DirectSound instead of the Microsoft GS synth. However, this doesn't fix games, and, as said before, the Windows scheme sounds along with some other program sounds, are wrong. Not affected by the problem are usually wavs played back through an external program, and sounds in games, sans midi playback.

I'm using the onboard audio of my motherboard, which is an Asus A8N32-SLI Deluxe. I tried uninstalling the Realtek AC97 audio drivers and reinstalling them using the latest ones I could find on the internet, but it did not fix the problem. I would just install a sound card, but I'm having problems with my case and the sound card won't even fit for some strange reason, considering it's a standard sound card.

However, when I completely unhook my computer and turn the power all the way off, undo the side and try to mess with putting that sound card in, but not get it in, when I put my computer back together, the sound will be normal again, but only for that session. Once I restart, it'll be messed up again.

This is a very strange problem, and I've done all I can do to try and fix it permanently, so now I'm here, asking for help.

Your help is appreciated!

For comparison, here's an audio file, played back the "wrong" way and played back the correct, normal way.
http://www.bashedonavan.com/wrong.wav
http://www.bashedonavan.com/correct.wav
(Sorry for the hissing, but that's another problem for another time.)