So I finally figured out how to find the Master sound class! I had to go to View Options → Show Engine Content in order to be able to select that in the sound mix.
However, it still isn’t working with the setup I listed above. I suspect it’s because the Media Sound Wave hasn’t been assigned a class? But when I open it up, it doesn’t look like the one you’ve posted (your soundwave.png above), it looks like this:
There’s nowhere to choose the sound class. From what you’re suggesting, they should be affected by the Master soundclass anyway, but that doesn’t seem to be the case, unless my BP setup is wrong in the first place…?