I am experiencing the same issue as this
user and this
I noticed my log on, log off, error sounds, usb sounds do not do not playback after a period of time. System sounds do not playback even when I preview them in Control Panel. All other sounds play fine, Youtube, Winamp, MPC-HC, etc. Just not the default Windows sounds.
If I reboot my system the sound comes back. The only game I've been playing recently is Battlefield 3 but I've tried searching BF3 forums regarding this issue and nothing's come up.
I'm running Windows 7 x64 my soundcard is the ASUS Essence ST (1.41 UNi drivers