I had several issues with HDMI on a laptop in the past, where audio would work until the laptop went to "sleep", and then resume functionality only after a reboot.
The issue would also occur if hdmi lost connection with the tv for any reason (I.E. it came unplugged, or someone flipped the light switch that controlled our entertainment system cutting off power).
Anyhow, my fix was related to the ATI/AMD drivers in windows 7, the trick was to go into the audio options in windows 7 where you would select the default playback device, choose the HDMI sound device (provided you can get it to show up in the first place, may require a hard reboot), view it's properties, and uninstall the driver from there. After uninstalling / rolling back the driver a couple times, it rolled all the way back to the default / generic / non ATI/AMD windows HDMI audio driver. This driver has worked flawless for over two years now. Not sure if this is your issue or not, but it's something to try as a last resort.
Posting from just above the generator.