@LennyO,
Alas it made no different whatsoever. With the slider for MRA set to zero, it's still full volume and drowning out YouTube.
Back to the drawing board.
Anyway, as the main reason for me creating this thread is now solved, and the thread title doesn't reflect these sound issues, I think I'll leave it there for now on this thread and start a new thread asking the volume question again or providing a solution if I find one with any of the third party apps.
Thanks all for the replies and sorry again for all the duplicate posts earlier.