Seeing that you're on Vista, I think I may know the solution.
Click on the sounds speaker icon (button right, to change master volume), and click "mixer"
Make sure that you have teamspeak open when you do this. Then, you will see teamspeak, as one of the volume levels. Make sure this is NOT muted, and make sure that it is turned up. As in vista, you can change the volume of each program seperatly (THANK YOU M/S)
Also - if this does not work, do you happen to hear the "player joined" and "switched channel" and such?