Did you run the OCC files:scan ?
No, I don’t known this command
- log on to your NextCloud with ncadmin account
- enable OCC Web app (https://apps.nextcloud.com/apps/occweb), it should be in Tools category
- launch it from the icon at top bar
- find your user with “user:list” (ID is at left, before the “:” ), let’s say “me@ecloud.me”
- run a files scan against your account with “files:scan me@ecloud.me”
- there’s no “exit” in OCC Web, just switch to another app
Hint : within OCC Web, you can get commands list with enter on a blank line, and help with “help command:subcommand”
About files:scan : you have to use it each time files are added/deleted/changed outside of NextCloud web or API (external storage, local storage using shell or ftp, …).
Unfortunaly it did not work, I did what you wrote and waited two houre but the music forlder did not sync with my phone.
Thans anyway for the great explanations!
It’s what I was afraid of : synchronization probably don’t work with NextCloud external storage 
In this case, I’ll do the mapping outside NextCloud, at OS level, hopping your provider will allow it …
You can take a look at https://www.looklinux.com/how-to-mount-samba-share-smbfs-in-linux/, especially the “Permanent mount” part.
Your files are in /mnt/repo-base/volumes/nextcloud/data/{user}/files/ …
Be aware that your NAS will have to be online when rebooting the cloud server.