I cannot open or play song's with anything but Files (default file explorer on AndroidOne
Android uses a media database ("Android MediaStore Database") which is used by most apps. If the file has not been indexed by the database or is unknown to the database it doesn't exists for the apps.

If you have copied a "tonne of music" this database may be a bit overloaded by indexing all the uploaded music files (or may be even crashed). 

Therefore I would first wait a while and then re-check if the media files are now visible in the player apps. If after some hours the files are still not visible, reboot the device. This usually triggers a re-scan of the media files.

Furthermore the actions I presented at the end of this answer also can be applied in your case.