I had a similar issue at one time. It was because of a sound driver that I compiled for upgrade. TeamSpeak apparently was looking for the wrong driver to use, and my solution was to reinstall TeamSpeak. If it is not a sound driver, recheck settings, and see if other applications still have sound.
Could there be a possibility that you have been muted by an admin? I am only guessing, it is hard to determine any issue with another's PC without physical access or remote access to make conclusive decisions regarding your problem.