What if you switch the volume mode to Internal by clicking the little icon to the left of the volume slider?
Great suggestion.
Internal produces no sound at all, but Application allows me to adjust volume without MC crashing. So that's great. I can use this as a workaround for now, and can at least use MC again! Thanks!
The video glitch with MadVR still remains but I found a workaround for this too...
First of all, bear in mind I have 4 rotated LCD displays in portrait mode (which is apparently significant to MadVR support) and a 5th which is a LANDSCAPE mode 50" Plasma TV. The TV is of course the main display for enjoying full screen movies from a couch, whereas the 4 LCD monitors are set up on a desk in a different part of the room in an office setup.
To create the problem I play a video, push CTRL + 6 which sends it to a FULL SCREEN display on my 5th monitor (the TV). This displays and plays fine until I click on ANY window back on one of my LCD displays, like a browser or any other program. Once I click, the display freezes, although the audio continues.
The video having frozen (even though it's still playing, as the audio continues), I tried double clicking on the video to bring it down to Windowed mode rather than Full Screen, and it plays fine, resuming video synced with the audio. For me, this window is also located on the plasma TV. If I then double click again to resume full screen, the entire screen becomes black. This didn't work as a workaround.
The only workaround solution is as follows, just in case anyone else has this problem:
After CTRL + 6 sends it to full screen on the TV monitor, I immediately double click to reduce it to windowed mode, then double click again to blow it back up to full screen. Then I can resume my computer work and click on other windows on my 4 LCD monitors.
In other words, I have to do this initially before I click on other windows. If I send it to full screen and then click on some other window and freeze it, I can't recover from it unless I sort of "reset" by double clicking back in the MC Playing Now window where it says "The display is detached. Double-click to bring it back." At that point I can CTRL + 6, double click, double click.
Hopefully this is a clear enough explanation for those using rotated portrait mode monitors with landscape TV displays as their main exported full screen display with MadVR HQ.