Then I would approach it as just regular file syncing. Syncthing is probably the most popular piece of software, though I haven't used personally used it over the Internet.
If you don't want to sync every file, perhaps your NAS has a native desktop file sync client? Or I'd run something basic like Filebrowser and let them download the individual files/folders they want, though it doesn't do syncing.
Imho easy mode is just throwing it on Plex and making sure your port is set up on your router for external access, then just sharing your music folder in Plex.
You do not need to port forward to share a Plex instance over the Internet. App.plex.tv manages the inbound connections automatically. All you need to do is manage invites to your friends. They log in with their email/password or with Google SSO to app.plex.tv and your content will be available over a secure connection with no port forwarding.