And the one package missing that makes MuseScore show up is: alsa-plugins-pulseaudio which wasn't installed.
I wasn't sure installing that was a good idea before, since I wasn't sure of the pulseaudio pipewire setup in KDE.
pulseaudio and pipewire are a bit confusing to me anyway.
So now MuseScore is 100% working the alsa-plugins-pulseaudio present.