If you go into Voicemeeter, look in the upper-right and click on menu, there’s an option to bind your volume buttons to whichever channel you want.
I had a problem with Discord not picking up my mic. Turns out if you give it a balanced signal, Discord assumes you’re giving it stereo instead, combines both of them and sends…not a lot. It’s almost like adding 1 and -1. My fix was to get Voicemeeter to pan my mic all the way to the left and have Discord pick that up instead. Worked a charm.