There is already a Music group by default, with most of the music file extensions added out of the box. It probably makes sense to use that, and add any extensions that are missing, rather than create a second overlapping group, which may complicate things.
Hover the mouse pointer over the format lock icon on the status bar. A message will appear telling you where the current format comes from. What does it say?
#6 in Folder Formats: Detailed Guide shows an example. The guide is also worth reading if you want to understand how Opus chooses which format to use when you change folders.