https://www.cnet.com/news/polk-will-have-first-non-amazon-speakers-to-support-alexa-multi-room/

MRM lets you listen to music on one or more compatible speakers in a home at once -- aka "Party Mode." You can say something like "Alexa, play System of a Down everywhere" and every compatible speaker on the system would play Bounce, simultaneously. MRM been available since August 2017 on Amazon's own speakers like the Echo and Echo Dot.

MRM competes with Google's Chromecast built-in, Apple AirPlay 2, Spotify Connect and manufacturer-specific platforms such as Sonos, Denon HEOS and Yamaha MusicCast. Sonos itself is also rumored to receive MRM capability, but Sonos representatives did not respond immediately to CNET's request for comment.