Apple Support is of little to no help on this issue. Amazing really, that they can't simply tell us where to find the iTunesMusicLibrary.xml files now that they have shifted from iTunes to Apple Music. I've spent three hours today between Sonos and Apple trying to figure this out. After consulting with Sonos, I asked Apple Support specifically to just tell me where the xml information is written so that I can add the folder to Sonos, and they said it wasn't an Apple problem and to go back to Sonos. I have been an Apple product fan since 1986, when I bought my first Mac. I'm a shareholder. I love Apple products. So this behavior is incredibly annoying.
Having said that, and being angry with the Apple for their non-support, I have at least a partial solution.
Sonos has already provided information on adding Music Folders by going to Manage/Music Library Settings and clicking on the "+" button to add a folder. The issue was not knowing where to find the xml files that were written when a Playlist was created. Apparently, in the switch from iTunes to Apple Music, which started with OS Catalina, the music files and the associated xml files for playlists were disconnected. Here's where I found the xml files.
In Finder, go to Home (your computer)/Users/your user name/Music/iTunes. Under iTunes is the iTunesMusicLibrary.xml file. I was not able to specifically add the xml file. Instead, I had to add the parent iTunes folder in the path just referenced. This worked. Once I went into the Sonos app to "Update Music Library Now" (under Manage), all my playlists have appeared.
One remaining problem is that I got more than my playlists. For some reason, I also have a folder that looks like a playlist called "Library," and another called "Music." They do not show up in Apple Music, so I don't know why they are in Sonos. Within them are many, many, many songs. I haven't gone all the way through, so maybe it's even all my songs. Maybe it's a subset. In any case, I don't know how to get rid of those folders. I plan to just ignore them at this point.
FYI, I'm using Sonos1 because I have several legacy Sonos products.
I hope this helps others.