In my own opinion, I think Martin should take time to look at the audio in vMix and recode it.
Then all other videos with audio should be limited to 50% and all Audio in/out be set via the Master Audio Input.
e.g. if No Audio input is selected for the master Audio, vMix should display a message that Audio need to be set. vMix should now look into that master audio to distribute as when needed...
Thanks.
Rottimmy
www.fusionplustv.com