The sound suddenly stopped on my Zune

I ride a bus into work, which gives me a chance of listening to podcasts on my second generation 80GB Zune.* I was listening this morning when suddenly the sound stopped.* I thought at first it might be the Zune freezing up, but it wasn't.* I could see the time advancing, so that wasn't it.* So then I tried listening to some music albums I've got on my Zune, and the same thing was happening; no sound at all, although the time was advancing.* The Zune is more than 50% charged, so I don't believe that is the problem.* And the weirdest thing is I've tried listening to any radio station I could, and I could listen to those.
It is acting like I've hit a mute button somewhere, but I've no idea where that would be on the Zune.* If there is some sort of mute functionality built into the 2nd generation 80GB Zune, where is it and how do I turn it off and on?
But if that isn't the problem, then what is going on??