Synology DSM 7.0.1. settings that do the job for me
File Services settings:
Minimum SMB protocol: SMB1 + NTLMv1 enabled
on a Synology NAS with DSM version 7 you need to set SMBv1 as minimum and turn on the NTLMv1 authentication protocol
Guest account with only read permissions to the music library folder(s).
SONOS Music Library settings:
Music folder path: \\synologyNASname\SharedfolderName\MusicLibraryFolderName
SONOS will convert the path name to //synologyNASname/SharedfolderName/MusicLibraryFolderName