Windows 7 - Periodic no sound from HDMI

Occassionally, I will get no sound specifically from web pages with what I assume
is an embedded sound file, but mostly from YouTube flash files. Even rebooting
or complete shutdown will not correct the situation. It seems to resolve itself
after a while.

Windows sounds continue to work properly as well as movie files I play through
MPlayer or sometimes Media Player.

I cannot track down what is causing the intermittent sound loss. Right-clicking
the sound icon in the task bar and selecting playback devices always reveals the
same setup:

Realtek HDMI Output
ATI HDMI Audio
Default Device

Nothing is changed since the original setup and troubleshooting only points out
that Speakers or headphones cannot be found which would be expected with an
HDMI setup.

This is really bugging me and I'm wondering if anyone else has experienced this or
similar problems.

When using the HDMI audio, the video card is using it's own built in sound device, not the PC's sound device. If the sound works OK with the regular PC sound (Realtek) and PC speakers, the problem is in the Video Card's sound system.

Youtube uses Flash and that has known to be iffy at times. Make sure you have the latest version of Adobe Flash. Some web pages with embedded audio use Windows media player extension and with the HDMI audio that may also be a problem.