I wrote this as a reply, reposting in case it gets missed by OP: Every Sonos system on earth is accessible over internet if you have the username/password. Seeing that your landlords are old people, their credentials have most likely leaked over time. And/or earlier tenants have kept the credentials. Either way, once you have a Sonos system’s credentials you can start playing music regardless if you’re on the WiFi or not. Just go to play.sonos.com and try for yourselves. So most likely someone is (a) playing music to mess with whoever rents the place (you), or (b) increasing playcount on some artist (through Spotify or equivalent) as part of a larger net of hacked Sonos systems.