Musescore mistakenly suspends pulseaudio on Ubuntu
By default Ubuntu has mawk installed instead of gawk and so Musescore suspends pulseaudio when it does not need to.
A temporary fix is to install gawk, however this should work out of the box.
Other people have had this issue:
Output of terminal when starting without gawk:
/usr/bin/musescore: 11: /usr/bin/musescore: gawk: not found