Select multiple files with different extensions > hold [Control] and right click > Get Summary Info > Open with (select all) > click [Change All...].
The files don't need to be valid, just have the extension you want to associate, so you can do something like `touch _.mp3 _.mp4 _.m4a _.mkv` then select those files and set them all at once.
The files don't need to be valid, just have the extension you want to associate, so you can do something like `touch _.mp3 _.mp4 _.m4a _.mkv` then select those files and set them all at once.