Weird problem with .aac files

Hello all

I have run into a very weird problem with .aac files. Obviously .aac files used to be the default audio format for iTunes - I am not sure what the current default format for the Apple Music app is? Anyway, I recently downloaded a couple of .aac files from two or three third party sites (ie not the iTunes Store). When I clicked them, however, they opened in the Books app for some strange reason! When I then changed the file association for these specific files to the Apple Music app, I got a security warning - but when I then confirmed that I still wanted to open the file, nothing happened. The file simply did not open. I then tried changing the default app for all .aac files,: the security warning then no longer appeared but the files still refused to open in the Music app. I redownloaded the files several times with the same result. What's especially odd is that the problematic files open and play quite happily in author audio players, such as the Vox app and QuickTime Player. What could possibly be happening here? Any pointers gratefully received