Permission for set a ringtone; "csrutil disable" does not help

You don't have permission and can never gain them because that folder is on a read-only volume that was used to create a "sealed" snapshot which is what is booted. That folder is not writable by anyone including root no matter how you attempt to do so.

Why do you have to modify a system file? There should never be a need to modify a file in the system area. Any locations that are expected to be modified already have links to the writable user volume. Modifying any system area is always dangerous and should never be done. Is there no way to configure the preferences in you home user folder to allow for this custom setting? Users should only ever have to modify settings within their own home user folder. I highly recommend you look for options to add your customizations to your home user folder.

Also keep in mind that if you do somehow make unauthorized changes to the read-only system area, that those changes may be completely destroyed after the next macOS update.

Now I tried to move that sound by hand in recovery terminal, but that terminal has no right either move the file or change the directory's rights.

It may still be protected by SIP in Recovery. Did you attempt to disable that in this attempt.

Apple is attempting to make it impossible for anyone but them to modify the system volume.

Even if you managed to change the file in that system volume, it may not be able to make a sealed snapshot from that volume since you modified it. If it can't, you won't be able to boot.

And, even if you changed the file, it might not create a new sealed snapshot until the next update, so you may not know if it worked until the next update.