A few weeks ago, I was in France. I took advantage of the occasion to upgrade my mother's computer to much more recent hardware, as well as her television to a new 46" LG LCD HDTV. The PC was setup with the final version of Windows 7 x64.
She kept her old PC monitor, a 15" LG LCD, with only a VGA connection, as well as her old Yamaha RX-V420RDS amplifier, which long predated HDMI.
The PC sported a Sapphire video card with an ATI Radeon HD4850 chipset and dual DVI outputs. One output went to the 15" LCD with a DVI->VGA adapter, and the other went to the TV with a DVI->HDMI adapter and a 15m HDMI cable.
The PC audio was connected to the digital coaxial input Yamaha amp via a 15m coaxial cable.
After I taught her how to switch back & forth between her 2 displays in Windows 7, my mother noticed a very odd problem : when she was using the TV as her display, all sound disappeared ! But when she switched back to the 15" analog VGA LCD, it came back. I was unable to debug the issue for her over the phone. But I still had some more time before flying back home, and determined the root cause of the problem in person : Windows 7 had seen it fit to automatically redirect all audio to the HDMI output of the video card when the display was switch to the TV !
This was of course not what was desired for her setup. The TV's audio was muted and we had no intention of using its built-in speakers. We wanted the audio to always stay on the digital coax output hooked up to the Yamaha amplifier . I fixed this in the Control Panel by disabling the ATI HDMI audio output device. I hope this will help someone else running into the same issue of lost sound when switching displays.