I understand that when you do a shutdown (or restart) the sound comes back, but does it just stop while you are listening? Does it do this on its own, or are you doing something at the time?
Also when did this start happening? After installing some software, such as a new game, or any type of system cleaning software, such as a registry cleaner, perhaps?
Sorry for all the questions, but this issue needs to be narrowed down as to when and how.