You haven’t specified which speaker you’re trying to attach. On the PLAY:1s I’ve messed with mounts for, it’s been a fairly standard screw, the same as a camera mount, or pretty much every other speaker out there. However, it’s been quite a while since I messed with all that, I don’t recall if it was M11 or M10, but a quick trip to your local hardware store should sort you out. 

Hi Bruce- thanks for your reply. It’s not the screw I’m after- I’ve got that. It’s the supporting plastic dowel plugs you push into the back of the speaker and the bracket you’re fixing it to. That’s what I need.

Not that I’ll be able to answer the question, but which speaker? Which mount? It may make a difference, and help others help you. 

You may want to call Sonos Support directly to discuss it, too, they may be more responsive than this community.