You can’t add them to what you have without first removing the Play:1’s. Then use the Eras as surrounds, and set up the Play:1’s in another room. 

The system doesn't support play1 and Era 300 at the same time?

Your current Home Cinema is known to the Sonos App as a single Sonos ‘Room’.  As @nik9669a states, to add two new Era 300’s as replacement surrounds, you need to complete the following steps:

• Open the Sonos App>Settings>System and select your Beam
• Scroll down and select ‘X Remove Surrounds’
• The two Play:1’s can now either be named as two new Sonos Room’s or a Stereo pair in a single Sonos Room.
• When you power on your new Era300’s the Sonos App will detect them.  Add them to your existing system and then open up Settings>System and select your Beam again.
• Scroll down and select Add Surrounds.

If you experience any problems, then please come back and we can talk you through any issues.

(I’ve just down the exact same actions, when upgrading my Sonos One’s to Era 300’s.)

The system supports it. Once you’ve (a) replaced the Play:1’s with Era’s and set them up as another room, or (b) added the Era’s as a stereo pair in another room, or two single speakers in two rooms, you can then use the “group” feature to play the same source to both/all rooms, playing in sync. This is the Sonos strength: syncronised, multi-room audio. But if you want Beam, Era’s and Play:1’s in the same room, this is not supported. The Beam is already the three front channels and the Sub gives you a  3.1 system. Then the surrounds (whichever you choose to use) complete the 5.1 setup.

Therefore I can configure the Era 300 as surround with Beam and Sub mini, and configure the two Play:1s as if they were in another room, but physically always leave them in the same room.  This way, using multiroom audio they will all be playing in the same room.

Correct  - I have a Beam gen 2 with sub and surrounds plus a stereo pair of Fives (plus now 1 Era 300) which are all in my lounge, but the Sonos app sees them as in different ‘rooms’

Just be aware that you will get lip sync issues when playing TV Audio, if you group another Sonos Room(s) to your Home Cinema set-up.

Yes, but when playing tv source there will be a short delay (approx 70ms) for the sound from the “second” room, which will sound like an echo. There has to be a buffer, so the speakers can all play in sync, and playing in this way is not how Sonos is intended. I often play tv sound in the kitchen, so I can still hear tv audio whilst out of the room but the room and speaker is far enough away I only hear one speaker room or the other.

I agree with the posts above. If you are just playing music then the various speakers will be in complete sync. It is different though with TV audio.

If I remember correctly, among the app options there is the function that allows you to delay the audio of the speaker

If I remember correctly, among the app options there is the function that allows you to delay the audio of the speaker

There is for increasing the delay but this won’t resolve this issue.

If you want a 7.1 system, which from your posts seems to be what you’re driving at without saying so: Sonos is the wrong solution for you.

Or, if you want separate front speakers so that you can position them for a wider soundstage, the same applies. Many have asked for this but (so far) Sonos have not delivered...

I hope Sonos gives the possibility to have a 7.1 system, it would be very interesting to use

Isn’t the setup of Arc + Sub + 2 Era 300s technically a 7.1.4 system? 

Arc is 3.0.2, with the addition of the Sub it goes to 3.1.2, then each Era 300 is 2.0.1 (a pair of them makes it a 4.0.2) which in the end totals to a 7.1.4.

Technically, yes. My interpretation of the OP and subsequent questions was the desire for physical addition speakers rather than virtual ones. 

Would be good if there was more freedom with setup, make it possible to pair 4 Ones or Era 100s to make a full 7.1 system. Should be doable through a software upgrade in the future. 

Fingers crossed. 

Likely would require an upgrade in various electronics to handle the change. More CPU, more RAM, probably a better Wi-Fi card, too. 

And for tv manufacturers to introduce a “video delay” setting, to help with lip-sync issues. 

As for the TV speech delay, I was wondering if it can be solved by connecting all the home theater devices through the Sonos Bridge?

What feature in a Bridge makes you ask this?

since the Bridge creates a Wi-Fi network dedicated to the sonos system I thought it could avoid the delay between the various speakers.  Of course, that's just my reasoning.

Bridge is old technology and would not remove the system delay. Nor would Boost, it’s “replacement”. 
 

As already explained above, Sonos has developed a system that enables multi-room, multi-speaker synchronised music. This synchronisation is achieved by introducing an initial delay in the streaming. When Sonos first developed the multiroom capability they had music systems only. Tv home theatre devices were introduced many years later, and have always had this limitation. For tv input this delay has to be minimised to avoid lip-sync issues. But when sending tv audio to grouped speakers ( ie not the Home Theatre “room” ) the delay is introduced. 

There is no way around it, based on Sonos current products and technology. 

Thank you