In my experience, you can get the album art from the Sonos device without having to go to a third service like Musicbrainz. That is, if the track has art associated with it or embedded in it. I have tested with Spotify (works) and my own ripped and tagged files (also works most of the time).
Here's my driver for doing this. I run the node-sonos-api on a RPi and also use the webhook functionality to cause refreshes by Hubitat on change from the server. You just have to set the virtual device DNI to the MAC address of the node-sonos-api server. Otherwise, you can just issue refresh commands to the driver.
This is a little bit rough around the edges and doesn't scale well to multiple speakers. If you like it and there is interest in using it system-wide, I could clean it up and release it for real.
EDIT: removed code snippet. See newer version of code below.