Why is all sound suddenly gone with HTC One S?

All of a sudden I have no sound with my HTC One S (running Android 4.1.1). No sound is heard when someone tries to call me, and, curiously, the microphone is also muted; the one who's calling me doesn't hear anything either. I also cannot play audio (it says something like *Cannot play this type audio file*). What on earth is going on? And how to solve it? PS: I have not spilled coffee or something over my phone; no unusual things happend to my phone, as far as I know.