It certainly sounds like the sound card is not all it should be, but that would not account for the .WAV playing on their own, or is there a typographical error that has made this ambiguous?
The crackling sound leads one to think it's a "dry" joint on the board but to track it down is very difficult.
I'd suggest you borrow a sound card from a pal and try that. If it works you should consider investing in a new one. If not, post back and we'll give it another go.