Is the sound actually going into mute mode or is it just cutting out. You can tell if you have the sound icon in your system tray, you can see the red circle with a line through when its muted.
If it is cutting out: The function of it cutting out maybe a physical problem. The system may not have enough juice to power the sound. Try playing the sound on a low volume. My other guess would be to upgrade drivers and test, then swap out sound cards.
I have a hp pavillion 7935 windows xp the volume mutes and unmutes on it's own. Does anyone have any ideas on how to fix this? I tried checking the settings in BIOS as a hp agent told me, then they had me do a full system restore, then they told me to uninstall the VIA'97audio controller(WDM) and it froze my computer and lost start and task bar so I then did a destructive restore and am asking anyone here for help. If you need more info I'll try to get it to you, I am not very good thou. Thanks