Also be aware, if you turn on the "Automatically add matched songs on this PC to your cloud collection" on any of your devices, then turn it off, the damage is already done.
Your Led Zeppelin albums are magically available on all devices but are reduced to one song.
If you have multiple devices like me (1520, desktop, laptop, Surface and an Xbox One) it seems like you have lost half your music and it could also be the cause of the album art issue as well (Not an issue I suffer with, in fact I now have album art on stuff I never managed to have it on before)..
After turning this function on, on the basis of "I wonder what this does", all of a sudden you have in my case a few albums that are mucked up and on all devices before you can blink and I spent a morning and part of an afternoon sorting it out and learning how it all works. Only then did I add my main collection of local music on all my devices.
So now my local music is just that, local, my cloud collection is made up of stuff I have added to my collection from my Xbox Music pass (I got mine last weekend and think it is well worth the money, and I was very sceptical about it).
Hope that all makes some sense.
Bob
I only have a few messed up albums, usually when I find one I archive it somewhere and then download the xbox music version (if available)
The easiest way I have found to fix it when its messed up, if you then want to ignore the matching part
Launch the app and turn off the matching
Go into options and delete the location where it looks for music (options, preferences, where to look for music)
fully close music and start it again
it will now only show what has been matched and is streaming only
from songs tab select it all and delete (double check it is all streaming)
add your music location back in preferences and it will re populate all your music back into the app
or alternatively (this is the way I have done it many times)
turn off the matching and fully close music
move your music folder somewhere (cut and paste to desktop)
start music and it will only show badly matched songs
delete all again here
fully close music and cut a past your music back to proper location
restart music and it will re populate with music without the matching