Make sure you have the latest version of your sound card. What sound card do you have at the moment? Also, was the sound card put in by you or did it come with your computer?
Did you check to see if your speakers are working in anything else, not just FireFox and iTunes? Restart your computer and when you log in does the log in music play?
Also, you say you already checked the sound on your PC so that must mean it is either your driver or your own error somewhere along the line.