I have been having similar problems.
All audio stops playing EXCEPT Xbox Music, that works fine.
All other audio just won't play, including texttapping, alarmsounds and so on.
Have also been in a situaton where all audio except XBox Music was distorted, it was just digital noise coming out.
I called HTC Swedish support and they asked me if I installed any apps and if so, I should try to reset the phe phone to factorysettings...
Actually did that but all audio went dead again last night, not sure how to go on.
Reboot helps but I don't want to do that once or twice a day.
Did you find any solution?
/Mattias