Every time I try to play a CD, Windows Media Player says it "cannot play the file because there is a problem with your sound device. There might not be a sound device installed on your computer, it might be in use by another program, or it might not be functioning properly" and I can't hear iTunes songs either.

So far I've tried reinstalling the drivers and installing DirectX but it still won't work. Can anyone help?

have you got 2 audio devices? (i.e. onboard sound and an additional sound card?
if so, try the other one (the one you are not using)
if not, try re-installing WMP
i will try and think of more ideas and tell you if i think of some

Hope this helps,
Mike