We also notice that if we leave the OTA TV station playing in MC and just turn the TV off that when we come back several hours later, we have no sound on the TV until we go back and manually close the last TV station played and then start a new station.
This may or may not be relevant but have the same loss of audio with other equipment, e.g. a set top box, that is connected via HDMI to the TV. So I'm suspecting it may be a HDMI handshaking issue. HDMI causes all sorts of problems!
I don't know if it would work but if your TV has a HDMI input that's sepcifically labelled PC, try using that one. Or if your TV has a DVI input, use a HDMI-to-DVI adaptor and use that instead (you may need to change overscan settings in your video setting on the PC when switching between HDMI and DVI).
The symptom I get via a STB as that if you put the TV into standby while leaving the STB on and then bring the TV out of standby, the audio disappears. Or if the PVR has kicked in to record a program while the TV is off - switching on the TV after the PVR has booted up will cause loss of audio. The solution is to reboot/reset the source device while the TV is on, as you have found.