First you need to make sure you have added the One SLs as surrounds.  Are they all listed as the same room? (Beam + LS +RS)  If not, see this link:

https://support.sonos.com/en-us/article/add-surround-speakers-to-a-sonos-home-theater

Hi. Ive added the two speakers separately but they are all in the same room. 

That’s grouping.  You need to follow the link I gave to add them as surrounds.  Otherwise they are just repeating what the Beam is playing, not surround sound.  Ungroup them and then add as surrounds so they are all in the same room.

Ok, thanks. I’ll give it a go!

I’ve done that but it’s as if the sound in now only coming from the beam in front. I’ve listened to the speakers and there’s sound but you don’t really hear it. 

If I adjust the volume from my phone it only adjusts the beam

It actually adjusts them all.  The Surrounds are now part of the room.  You can adjust the individual level in the Settings for the room. 

Unless I actually go up to the speaker, I can only hear sound from the beam in front of me! I can’t find where I adjust the speakers since they have been added as surround. I notice that there is ‘stereo pcm’ is that correct?

You want the input to be some form of Dolby Digital - 5.1, DD+, Atmos.  Try different settings on the TV.  If you have Passthrough, use that.  Bitstream often works also. 

I’m lost with that! I’ve checked the settings on the TV. Anything here I should be adjusting?

I found ‘Bitstream’ so clicked on that (whatever that means!). 

And…? What sounds are you now getting from the speakers? What shows instead of “Stereo PCM”?

Hi. The sound seems better I think! This is what is showing now

As it says, you now have the correct signal reaching the Sonos system. Remember that the rear speakers are for effects so they won’t always produce sounds. And if the program you’re watching is only broadcast in stereo they will again remain quiet. When streaming music from Spotify or wherever they will also play the stereo sound along with your Beam. 

Thank you so much for explaining that! Really appreciate the help and knowing it’s set up properly now

There are some folk on here that go a long way to help newbies. Always good to see them getting thanked too.  

Any more questions or issues, you just need to raise a new topic

I may have missed it above but there are two modes for your system, TV and Music.

In TV mode you are pretty much stuck fiddling the levels with the source having most of the control of the sounds.

In music mode you have the level controls but also a choice of Ambient and Full modes. In Ambient Sonos fakes up low volume rear sounds to add a bit of ambiance to the front speaker. In Full the surrounds mirror the front L/R channels giving you much more sound.

I like Ambient for general listening but for parties or when I’m not in my usual listening spot Full is my preferred option.

Hi. I’m back again! Thought I was sorted - switched to Bitstream on my TV and Dolby Digital 5:1 was showing on my Sonos app - but today, although the TV still shows Bitstream, on my Sonos app it’s gone back to Stereo PCM. How do I get it back to Dolby? Thanks!

Sometimes the source is going to be stereo.  Play something you know to be DD 5.1 and see if it changes.  By the way, most YouTube videos are stereo only, regardless of what they are listed as.  YouTube downconverts to stereo when videos are uploaded, except for  the ones in their pay service.

Thanks!