The sound coming from my discrete ASUS Xonar 5.1 card and my onboard audio for my ASUS P8Z68-V have stopped working immediately after I installed a new device into the PCI-e slot on the motherboard: the AVerMedia AVerTV HD DVR.
A shot in the dark, but many times installing a new device will cause windows to reset the default audio device (even when not installing an AV device to the system). Go into your audio devices and disable everything except for the card you want to use, and set that to default.
I installed all the drivers, then disabled all the devices besides the sound card (I think... what should the sound card show-up as?). Only one of the four devices was "working," the back-panel onboard audio. However, there was still no audio after I connected it to that port.