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Scroll down in the "General" tab of settings to see the music bar and adjust the volume
The music bar is not immediately visible when you open the settings below the "Sound" bar.
Fixing the problem
Scroll down in the Settings menu to see the Music bar below Sound and adjust the music or sound volume. The game team are looking into adjusting the Settings menu UI (interface) so that the Music tab is visible when you first open the Settings screen and you know to scroll.