Musescore is lacking in accordion sounds. It has a built-in soundfont that’s barely OK for accordion sounds on playback. It does, however, accept some virtual sounds.
Musescore now comes with Muse Hub, a facility which gets downloaded first, if you wish, but contains many sound packages that have a decent quality. Some are free. Once you download and install Muse Hub, you can download and install Musescore, Audacity, and other sound apps and packages from within Muse Hub. Unfortunately, there isn’t a sound package for accordion or other free reed instruments — yet.
Once inside Musescore, each staff becomes a ‘track’ in the mixer, and each track can be assigned to a sound from Musescore’s built-in soundfont, or from one of Muse Hub’s instrument packages (assuming you’ve downloaded it,) or another virtual instrument (again, assuming you’ve downloaded that.)
I’ve found a few things that might help. One of the problems with using that built-in accordion soundfont is that treble and bass play back at the same volume. That problem has been solved in version 4.4. It’s now possible to select separate dynamic markings in each staff of a grand staff: for example, mf in the treble and p in the bass, and it will play back that way. Or, you could use the mixer. Or, if you spell out the chords, you could put staccato marks under them and hide the marks. (By the way, if you use a Roland accordion as an input device, there’s a method to get AAA bass notation and have it play back correctly, but that’s too complicated to post here.)
In the meantime, I’m going to lobby the Muse Hub people to create a free sound package for free reed instruments.