I haven't touched these settings but it is purely a matter of personal preference.  They control the relative volume of front and back. TV level applies to TV sound and Music level to music. Just play around with them to hear the effects. 

Just to slightly complain about the labeling, it’s technically the different inputs. Music is the wifi (streaming) input, TV is whatever is coming in on the digital input, which normally would be the TV set (but doesn’t have to be).

:)

I guess I should explain why that’s a complaint from me. People could easily be playing music on their TV, such as YouTube, etc. And then fiddling with that volume slider labeled Music wouldn’t make a difference.

Yes, I’m extremely particular. Why do you ask? :)

Yes. I would also like to know the optimal level for tv surround level and music surround level percentage :)

There isn't one, as explained above. Set it how you like it in both cases.

I understand that you want to set it as you see fit, but I’m curious as to how others set there’s.  I wouldn’t of posted this if I just wanted to do it my way lol.