If its on a laptop/netbook, is there a chance that you accidently hit the key combination to mute the sound?
That is not always shown in the volume icon.
Further, if you go the the menu, multimeida, is there something like the ALSA-Mixer?
I had once an issue with it, as it droped to 0, while the volume level was to the max but no sound was to hear.
If none of the above applies, whats the output of:
for e in sound media;do dmesg|grep $e;done