you say you checked the sound mixer, if you run the game in windowed mode, it sometimes allows you to see for a split second, if you switch directly to it, could you possibly upload a video, or take some scrrenshots, and maybe add a dxdiag, might be something wrong with the audio output of the pc. also, check under ingame audio settings, and move a slider around, some sound should start playing, if not, its either not producing the audio, from the game, or windows sound mixer, is somehow blocking the audio from getting out the speakers.
another possible cause, is that it for some reason, uses another audio output, which is not in use. i suggest disabling all outputs not in use.