Hi Paul,
I could be wrong but the fact that Sonos has to attach to the B"'s NAS then index this as a library (which takes ages), would indicate to me that Sonos is NOT using the Be "Scan Disk" database.
I would suggest that there may be some "bad" track content in the B2's music folder that may be causing the problems described. AT LEAST this should be ELIMINATED as a possible cause.
To trouble shoot this you need to find/remember the CD that was causing the problems and see if this will play OK when listening to it through a directly connected B2 output (some headphones plugged into the back would do!) and or
Look at this album by attaching the B2 NAS to a PC/Mac and looking at the track file sizes with the native file management tools.
Fred