From the description with the built in (probably passive speakers) my guess without having seen the connections in the closet would be that in every room there is also a MCL relay box

Location: Denmark

Whoever did the previous paint job didn't even bother to pop them off the wall.  I *hate* little rectangular brush strokes in in the middle of a roller-matte-textured wall.  Which of course isn't what you are asking?, sooo... IMO trash them all since you don't have any of the central equipment.  What you are looking at is the equivalent of Xantech remote-IR receiver/transmitters, only better-made (but still aged plastic switch parts) and at the B&O IR frequency, incompatible with other brands.  Maybe keep their wires (which hopefully also go to the central A/V room, you didn't say?) and use them for DC (USB) power supply to wall-mount a bunch of iPad Mini's -- which you will use to control your Sonos Amp's, all of which will also be back in that central A/V closet.  Home-run speaker wires is great, everybody seems to want all the smarts out in the room but it's not necessary or even always the best quality.  Replace the in-wall speakers on your own pace, as you discover foam rot or place your own furniture in front of the supposed "perfect" locations the previous owner chose.  Or if wired iPad Minis are just too much (they will cover those paint marks completely!?!), then repaint and stick up Lutron Pico Audio's either ganged up with the light switches (iff Decora) or separately just to cover the holes.  Then you will have the same button control for your new Sonos Amps, though not IR retransmission of course.  (Thus the reason for selecting iPads, source selection was part of the original B&O IR setup via handheld BeoLink remotes.) It's a B&O forum, so hopefully someone has a brilliant 20th-century idea for filling in your A/V closet instead of my wimp-out answer.  (If death was recent, search eBay listings for "ten great Bang & Olufsen SpeakerLink 'passive' kits plus a BeoCenter".)  The stuff really does look cool, nicks and paint drops notwithstanding -- but don't mount B&O Halo's in each room just because the previous owner left his old keypads there...  Nevertheless "audio right next to the light switch in every single room" is luxury, so I'd keep *something*.  As would most others here I'll wager.  Good luck, and please post what you decide to do ultimately!  For that matter, a photo of the current state of your audio closet would not be amiss -- might trigger great ideas from people less didactic and penurious than me!