i thought thx gives out some good troubleshooting guides on why you get no sound on your speakers...
Here is the pic of volume control...see if both Play Control and Wave/MP3 volume "Mute All" is not check (black circles).
And see if both knobs are turn all the way up (red circles). My guess is turning all the way up gives out the most loudest and dynamic sound...
but since i uses headphone sometimes and it gets too loud, i turn it half way, but if you are listening through the PC speakers, I believe turning all the way up give best results...