intermitint sound

my computer ,not very old and was working fine but the last week the sound keeps stopping working it will stop in all my games and in windows will say....media error:cannot play back audio stream:no audio hardware is available......now if i restart the computer it comes back working as normal but then, if theres no sound on computer for 20 min it stops working again....reinstalled drivers and updated amny times but no change.....pls hlp me its drivin me nuts. Also i play a game called world of warcraft and since this startred happening, everytime i log into game i get the user agreament page which your not suppose to get unless youve just installed the game or downloaded a patch, very odd. I have a pcchips motherboard with onboard sound (realteccAC'97 audio controller). I even tried a new pci soundcard but same result. Ive checked device manager for problems but was find except one time when yellow question mark next to audio controller i updated driver it went away but still having same problem