Please specify the steps you've already taken, so that we can rule out things.

You mention that you have trashed the Preferences.

Have you taken other steps?

Have you e. g. tried changing the file name extension from .mus to .musx ?

It seems that Myriad's "Melody Assistant" software, which I recently installed, is "commandeering" the .mus files. I have selected a Finale file with the .mus suffix and specified Open in Finale, Change All. It then shows Open with: Finale.app (default) (26.2.2.493). But at the top of the Get Info screen, it shows Kind as Melody/Harmony Assistant.

I zipped the Melody Assistant app and deleted the original. Now the Finale .mus documents show as Kind: Finale Legacy Notation File as expected.

I don't know how to unassociate Melody Assistant with the .mus suffix. I have emailed Myriad support for further assistance.

Tim

Select a Finale file icon, read its infos (Cmd-I), and at the bottom (Open with), select the program that will open the file and all similar ones. This usually works (on a Mac).