I mentioned this in an older thread on this same topic But I just experienced this today (about 20 mins go). I think that it had something to do with using the audio out on the phone. I did that today for the first time and shortly after experienced no sound. I only realized it becasue I saw a missed SMS and did not hear an alert. Soft reset cleared it up. Not sure if this is a bug in the software or what but I dont really like the idea of potentially missing important calls and or texts.
EDIT: Ok.. about 20 mins after soft resetting the phone I am having the same issue. Possibly time for a hard reset? The latest piece of software I've installed is stream XM from a thread over at XDA:
http://forum.xda-developers.com/showthread.php?t=450259