[Blumi]

MobileSheets most likely did not have access to /storage/6434-3134/Music during the sync, as it's outside the storage location. If you set the storage location to /storage/6434-3134/MobileSheets, for example, it will prompt you to provide access to the SD card, and you have to pick the root of the SD card (/storage/6434-3134) and then MobileSheets will be granted write access to the SD card by the OS. Then the folder mapping should work.

Mike

Thanks a lot for your quick response. I did as you suggested and yes indeed, the path was mapped to /storage/6434-3134/MobileSheets successfully. Of course there wasn't any audio file. 
Then I deleted the newly sync'ed song on the client, changed the audio location back to /storage/6434-3134/Music (was asked again to grant permisssion to this folder) and started the synchronization again - but for whatever reason no mapping was done. The path to the audio now was /storage/0000-0000/Music (the value taken from the server side).

This really is a miracle to me, maybe there is something special about the folder named 'Music' on Android devices ?
Anyway, it's not a big issue, the vast majority of songs is in a correct state now on the client device and finally I know now how to replace invalid paths using the ' Fix Broken File Paths'-function.
After all, it is only an emergency copy on my smartphone.

Thanks anyway for your continued and quick support!!!