So, I followed the advice, guides and thread replies from over a year ago to no avail. I have rebuilt my files in my nas, and suddenly by chance noticed two particular missing albums out of 7 by that artist were wma lossless. So, I ran them through the j river converter to FLAC, and voila, they are indexed and appear in my music library. I'm sure the sonos compatible file types includes wma files, but in my system they're definitely not. Worth trying if like me you were about to ditch the whole sonos thing out of frustration.