Alternative solution: Buy a little box named "HDMI audio extractor" that allows you to extract the audio signal from the HDMI source (Google Chromecast) and connect it to the AUX IN of a Sonos Play:5.

By that your device thinks it's streaming to a TV and the cast button is shown as usual.

Pro:

• you can control the output through multiple devices instead only the one casting through Google Home
• it streams directly from the internet without passing your device

Con:

• it uses much more bandwidth because it streams the video as well (maybe you find an extractor that fakes the EDID information of the "display" to 480p so it's video quality is reduced as well)

Alternative solution: 
Buy a little box named "HDMI audio extractor" that allows you to extract the audio signal from the HDMI source (Google Chromecast) and connect it to the AUX IN of a Sonos Play:5 or Sonos Connect.

By that your device thinks it's streaming to a TV and the cast button is shown as usual.

Pro: 

• you can control the output through multiple devices instead only the one casting through Google Home
• it streams directly from the internet without passing your device
  
  Con:
  
• it uses much more bandwidth because it streams the video as well (maybe you find an extractor that fakes the EDID information of the "display" to 480p so it's video quality is reduced as well)