I gave up using Libby with the Sonos app until I recently found an easy solution.
Solution: Instead of using the Sonos app, to listen to Libby audio books, I now use the AirPlay feature.
Requirements: Sonos speaker with AirPlay capability plus a device, like iPad, with AirPlay capability.
Steps: Play the audio book through Libby, on an AirPlay device, as usual. I use an iPad. Swipe down and leftish from top right of screen, select AirPlay (triangle with lines above). My networked airplay Sonos speakers are available. Select at least one.
Result: I hear the audiobook throughout the house.