All file references are stored in DB inside upload_file table/collection.
So let’s assume you copied a file called test.mp3 to /upload now, you should create a new record in DB for reference like this:
Now it is visible in Strapi, and I can use it:
To automate that process, I would recommend using a watcher, that detects all the changes inside a folder, so you could trigger a function that creates them in strapi DB. You can find a lot of modules that provide that functionality to watch for changes in a specific folder, in your case /upload/music.