As Google home, Alexa, Sonos and other speaker offer multi-room audio, here is a different approach.
I recently renovated my flat, and wanted to hear music in every room using better quality audio. To generate quality audio I decided to use in-wall 8" woofers, and a 35W amplifier. I found the IQaudio project that adds a DAC and amplifier to a raspberrypi. This bundle creates good sounding audio, but then comes the control issue. The raspberrypi connects to the home network via wireless or Ethernet. Basically I use MPD to play music on the raspberrypi, so I looked for a web interface to MPD. I found Rompr does this, so I can open a browser on my smart phone and control the music in each room. But I also wanted smartthings, to start/stop music for the rooms I am in. So I wrote this device handler for smartthings.