Can't uncheck the Play part only checkbox.
"Musescore "solos" the Part you're viewing during playback—i.e. no other Parts sound. This is because the Mixer’s “Play Part Only” box is checked by default.
Simply uncheck the “Play Part Only” checkbox if you want hear all play enabled tracks while viewing a Part."
Except in MS3 it is greyed out and cannot be changed. So I loaded 4.0 and that does not even have the check box and it appears to be impossible to hid parts on the score.
I need to show 1 part and paiano whilst still hearing the other parts in oder to create training videos.