Just looking at this briefly, it looks like you’re sound mixes aren’t applied to any sound classes. If you want your sound mix to apply to the master sound class, you need to specify the master sound class in the sound mix.
I think you were confused when we said it would work automatically for sounds. New sounds will by default play on the master sound class. What that means is your sound waves, sound-cues, etc, will look like this by default:
I’m not 100% familiar yet with media player, but all it is is a “procedural source” that is a derived class of USoundBase, which is the thing that does this default behavior with sound classes, etc. So it should just work with the sound mix system.