Happens to me a lot on my Droid Bionic
I went to the Verizon store. The Rep told me it happens on his Razr. He said to plug in head phones or an HD cable and it cures the problem. I said what if I am where I don't have a cord. He said just power down and back on (no need to pull the battery)
When it happens to me it is everything that uses the Speaker, Pandora, Slacker, you tube, mp3 stored in my phone. I can tell when it happens by placing a speaker phone call, I don't hear the button tones. I was on a very important call on speaker so my wife could be in on it. The speaker stopped. They could hear us but no sound on our part. He had to call us back on my wife's iPhone. One time it was off so I turned on every sound I could think of, my music, slacker, Pandora etc. No sound, then I got an incoming email and the notification tone "woke it up" Then I had all that stuff playing at once
The Verizon rep said that they know about it and to expect a fix at some point.