Many users report that their Music Library has disappeared.
The reason for this (in most cases) is probably due to the so-called SMBV1 file sharing protocol being removed. Many older systems use this protocol.
File sharing is now via SMBV2/3
Many older systems cannot be updated to use the new protocols. Users will need to purchase new file sharing devices.
The reason for this change is based on security concerns where the smbv1 is regarded as a security risk. This is true in so far as a system has already been breached via another external route (there have been no recorded instances of a SMBv1 impacting Sonos systems in the last decade).
Essentially, the change which has affected so many is another ill-founded and poorly implemented characteristic of the S2 wonder update.
Never mind, you can now listen alone to your Arc soundbar by purchasing the new $500 headphones. It's not all bad!