I have a "home theater PC" hooked up my main television at home running Windows 7 Home Premium with Windows Media Center. I also have a logitech harmony remote and an media center remote receiver that came with my AverMedia TV tuner card.
I have found that the system audio will automatically mute itself when one of two things happens:
1 - I use the harmony remote to start a new activity (thus it powers down the current activity)
2 - The PC is left idle for a length of time.
In both cases the PC appears to be in a 'sleep' mode and can be woken back up using the 'Power Toggle' command programmed into the harmony remote, but the system audio gets muted. It's annoying because the remote is configured to map the volume control buttons to the audio of the television rather than the audio of the PC meaning that I usually have to find my wireless mouse to go and un-mute the PC.
I have a feeling this is somehow related to my power saving settings, but haven't figured out how to make it not automatically mute itself when going into stand by.