I'll upload this package to the AUR. But how about adding groups=('soundfonts'),
The use of a group for AUR isn't very useful because the user, to install the whole group, needs to individually download each PKGBUILD and build/install each package. Maybe one of these automated AUR building script is smart enough to scan the list of AUR PKGBUILD and find/build/install all PKGBUILD belonging to a certain group.
However, you should still add the groups=('soundfonts') as it will be useful if the PKGBUILD is to be included in a binary repo (personnal, community).
I just wanted to point out that it might not have the convenience that you thought.
and change pkgname to soundfont-unison?
Yes, use soundfont-unison as pkgname.