Hi @roger7
Welcome to the Sonos Community!
Only 6 music albums out of 300 to 400 are held on my computer. The rest are in ICloud.
The 6 are the only ones to appear in my Sonos music library after updating it.
This is fairly telling. Please copy the tracks currently stored in iCloud to local storage (to the same location as the 6 albums that do show) and re-index from within the Sonos app: Settings » System » Music Library » Update Music Index Now.
I hope this helps.
"Common sense is the collection of prejudices acquired by age eighteen." - Albert Einstein