To power passive front L/R speakers you would need the Amp.  Unfortunately, the Amp doesn’t do Atmos, so the Era 300s cannot be used.  You would need to purchase Ones, or the Era 100s for surrounds.

Okay, thanks. Could I use 4 Era 300s? Two front and two back?

No, the Era 300 doesn’t have an input for TV.  Only the Amp and the soundbars have HDMI/optical. 

Thanks, so I could go for two Era 100s as rear speakers and two Ones at the front? Or just 4 x Era 100s?

When combined with a TV you need a Sonos soundbar or Amp. Neither One’s nor Era’s can be connected to a TV or be used as front for TV sound.

Thanks, it is to be used with a projector.

The Sonos setup is designed primarily with TVs in mind, but can be used with projectors.  Either your projector needs to have HDMI-ARC capabilities, you use a device that generates an HDMI-ARC signal between the source and your projector, or you covert the audio to optical...which limits what audio formats you can play, no atmos.  Either way though, the audio has to connect to a Sonos soundbar or amp powering the front speakers.  Lots of ways you can setup this up with Sonos, or with other products, but it’s hard to give good answers without more information.  What’s the the room size, can you put wires behind the walls, do you mind wires outside the walls, what are you hoping to achieve in terms of audio quality?

BTW, they do make acoustically transparent screens, so you absolutely can place speakers behind the screen.

Thanks for your response. The room is around 3m by 5m and the screen size is 110 inches. I was planning on feeding a blue ray player or Firestick into an amp and then an HDMI cable into the projector. Wiring is not a problem in walls or the ceiling. I am already fitting Sonance ceiling speakers into another room and could do the same. Looking for good quality surround sound, doesn’t have to be Dolby Atmos if that is not achievable.

Honestly, if using a projector and you can wire behind the walls, a traditional AV receiver is a better option than Sonos.  That’s not a knock on Sonos, it’s just not the best product for this situation.  Even if you want to have to have the convenience of playing streaming music through Sonos, you can connect an AV receiver to a Port to acheive that.  Sonos even has a ‘works with Sonos’ program on some Sony/Onkyo/Pioneer receivers so that you can control the volume of your AV receiver via the Sonos apps.  That’s definitely the direction I would go in your situation.

So an Atmos speaker you cannot use with a TV for cinema?

It can be used as rear speakers (covering rear, side, and height audio) when bonded with an Arc or Beam Gen 2...which will connect to the TV.