It looks like .md files are associated with the Musescore software for whatever reason on your system.
Obsidian can’t be the handler or default app for .md files (it operates on files inside a vault), so you’ll need to associate .md files with an app than can handle them.
In the Finder, cmd + i (“Get info”) on any .md file.
I’ll just repeat what ariehen said, but in a different way.
It doesn’t matter what the default “Open” is set to. The files are still what they are. And you are free to “Open With” those files with whatever application is able to open them.
If you open them by default, they’ll just open with whatever application they are set to open with. That does not change the content or the nature of the files. (Unless you save the file from insisde Musescore. Then you might damage the file.)