Decided that I'd go for some small wallmounters mounted on the ceiling, can get ghost wire from Amazon. The "flat cable" I managed to find was still around 2mm thick and not that much cheaper than the ghose wire which I can just paint over.
Only problem is... budget

need a new AVR and some other things came up. Will probably only be able to upgrade next year (which sucks because some good black-friday pricing around).
Still... would have been fun to get battery powered bluetooth speakers (or wifi connected e.g. heos or sonos), no wires, just pop them down to charge every now and then.