turns out, since the last update any music playing is stopped whenever I open ML. I submitted a bug report, and got the reply that this was by design.
Well, @Community-Managers answer me this question: As a user who loves to listen to music, who chose to mute monster legends, how exactly does it improve my playing experience if I'm now denied to listen to my favorite music while playing?
Next step would be to get rid of the options to mute sound and music, too? Jeez. (For the record, I spent my last trips to work reading and not playing ML. Could get used to that, too.)