No. Not if you’re expecting 5.1 sound from just the Era 300.

@Rich_10 That setup will not work. You need a Sonos home theater device like an Arc or Beam (Gen 2) connected to your TV. And ideally you need a TV equipped with HDMI eARC to play the lossless audio formats used on Blu-ray audio discs. You can then add a pair of Era 300s as surrounds to the Sonos sound bar.

If you use the Era 300 as rears, your primary source will be the Arc. It is not possible to only get the 300s to play the music. For the moment, it seems like you can only play music with Dolby Atmos with Apple Music on the Era 300.
There seems no good way to get sound from a Bluray player to the Era 300 if you want to use them for music with Dolby Atmos. I was thinking about pairing the Era 300 and then grouping them with Arc. But I believe that the Arc will send the music in Stereo and doesn't obtain the Dolby Atmos signal.

The Amp has an HDMI port, but it seems only to support stereo and cant be used with Era 300 as front speakers. 

The Amp can also do 5.1 DTS and Dolby Digital if bonded with rear speakers. It just cannot play Atmos. I’m not sure what the format is BD audio disks play.