I would have thought that you could use the name of your server (DiskStation) or the corresponding IP address. The former is generally more robust (IP addresses may change between reboots of your Synology server).
Then you also need to specify the share name. If the shared folder is called ‘Music’ then //DiskStation/Music should work. If you actually want the Music subdirectory that’s in a shared folder called ‘MyStuff’ then //DiskStation/MyStuff/Music would be the answer. You will need to provide the username and password if your shares are secured with one.
If you restart the system, after it reboots you should see a message on the screen saying how many files it’s found during scanning.
However, it’s easiest to debug this if you enable SSH access and log in to PiMusicbox so you can manually run the commands that are executed during startup. Assuming the username and password are ‘fred’ and ‘topsecret’ respectively:
mount -t cifs -o sec=ntlm,ro,user=fred,password=topsecret "//DiskStation/Music" /music/Network/
Then see if your files are now available:
ls -l /music/Network/
If that worked then update your PiMusicbox settings to reflect that:
mount_address = //DiskStation/Music
mount_user = fred
mount_password = secret
If you don’t need a username/password then maybe try
mount -t cifs -o sec=ntlm,ro,user=guest "//DiskStation/Music" /music/Network/
Or
mount -t cifs -o ro "//DiskStation/Music" /music/Network/
Then when you next reboot the server should be mounted automatically and the scan will find the music files.