I am not a Mac person. It is possible that Big Sur no longer supports or has disabled support for SMBv1 file shares. The SONOS library requires SMBv1 support.

Me too - spoken to apple and they say it’s a sonos problem, but Sonos can’t fix it either!

I’m really fed up with it - have spent about 5 hours trying to get Sonos to play an existing playlist using files in my downloads folder - absolutely refuses!

Not a Mac user myself but have you checked the Mac’s “Security and Privacy” as described in this thread?:

“Comparing “System Preferences”  in Big Sur OS on the MacBook and the previous OS on the iMac I noticed a difference in “Security  and Privacy”  and the two “Firewall” and “Privacy” tabs. In Big Sur, Apple moved SONOS stuff from one tab to the other.”

Thanks for your reply.

Yes is the short answer - have fiddled with security settings, sharing settings, firewall settings. Rebooted multiple times - have literally spent hours on it. 

Apple blame Sonos - I blame Big Sur which has many many gremlins!

SMBv1 has security issues and some companies have removed support for SMBv1. There are torturous schemes to install SMBv1 support into Windows 10. Reading some online accounts of attempts was interesting. I’d read an account of someone who outlined a successful, magic scheme, only to be followed by another account declaring that the scheme does not work, then giving their own scheme, followed by a ‘fail’ rant and on and on.

Personally, I’m not keen on storing my music library on a computer, I prefer storing it on a NAS (Network Attached Storage) drive. The computer can then crash and burn, while my SONOS music continues uninterrupted. Very old NAS drives will support only SMBv1 and this is great for the SONOS library, but the latest versions of Mac and PC operating systems will crab. Replacing the drive with a new drive that supports both SMBv1 and above is a relatively easy and inexpensive solution. Personally, I was struggling with an old SMBv1 drive, trying to decide what to do, then the drive died and my problem was resolved (yes, I had a backup).

I briefly struggled with this in a household that used both Mac and PC (his and hers). It’s contorted, but possible on both Mac and PC to temporarily convince them to deal with an SMBv1 drive -- long enough to extract the files. After a while both operating systems will revert to blocking SMBv1 access. There may have been a more permanent solution, but I did not follow through to the end of the trail.