If the source signal is 2-channel then the surrounds are sent a synthesised ambient sound derived therefrom. They wouldn’t normally be totally silent unless the content is, say, a news channel.

In the case of music (not TV) you have the option of the surrounds producing ambient sound or reproducing the full stereo signal. 

Thank you. I will go ahead with the purchase of my new speakers.

I suspect that Dolby Digital 5.1 soundtracks usually come from DVD or blu ray players.

Also I can (sort of) hear the advantages of Dolby Atmos when playing songs on Amazon Music Unlimited or watching UHD movies on Sky Q so I can only assume that Dolby Atmos will sound even better with surround speakers?

I have also gone ahead and ordered a pair of SL speaker stands to complete my home theatre system.

@MikeE67 5.1 audio is commonly used on DVDs, Blu-rays, and all of the major streaming apps like Netflix, Prime Video, Disney+, HBO Max, Hulu, and Apple TV+.

Adding surround speakers will make a pretty dramatic difference when listening to 5.1 and Dolby Atmos audio. It will give you a much more immersive home theater and music listening experience. You will not regret adding them to your setup.

GuitarSuperstar- thanks.

I am a member of Which? and I read that the SL surround speakers are a bit of a hassle to set up on the app.

I found it a breeze to set up my Arc and Sub so was a bit disappointed to hear this but I will persevere!

Sonos surround speaker are no more difficult to set up than any other speaker. 

Adding surrounds to the Arc is a simple process. Just follow these instructions:

https://support.sonos.com/s/article/2927?language=en_US

Thanks to you both for your feedback!

I guess I have nothing to worry about then.

I was going by this:

I set up an Arc, Sub and two SL surrounds back in December.  It was 1/2 hour from unboxing to fully set up, and that included drilling for and mounting two shelves.  Adding the surrounds took 30 seconds, tops.

I have had a few days to assess my surround speakers and I have to agree with you that I don't regret adding them to my system!

Just to clarify that I now have a Dolby Digital 5.1 system with the 3 channels on the Arc (left, right and centre) and the 2 surround speakers making 5 and the Sub is the bass i.e. the 5.1 ?

Yes, that is correct. :)

You actually have L/C/R  SL/SR  Sub, plus Left Upfiring Atmos and Right Upfiring Atmos, which is 5.2.1.

I thought it was 5.1.2?  No?  speakers.subs.atmos channels.

ToMAYto, ToMAHto. 

First, I laughed, then curiosity hit me. According to this website:

The format is "x.y.z" where:

1. "x" is the number of channels or speakers at or near ear level,
2. "y" is the number of channels or speakers dedicated to low bass reproduction
3. "z" is the number of overhead channels or speakers.

If a system does not have any dedicated height channels, then the last digit is left off.  

Much more on that link, though...

Hi.

I don't know if it is a 5.1 or a 5.1.2 system but I do know that my system sounds awesome especially Dolby Atmos which I can say that I can really hear directional sound now.

If anyone reading this has doubts about getting surround speakers I would say definitely go for it, I know it's extra cost but it really is worth it.

I know it's down to personal taste but, generally speaking, what height should my One SL surround speakers be set at?

Below ear level, at ear level or above ear level?

I have bought 2 adjustable height speaker stands and I currently have them at ear level.

Also I have placed my surround speakers approximately the same distance away as my Arc and Sub.

Is this ok?

Cheers  

Ear level whenever possible.  Mine are at ear level when sitting up, slightly above in full recline.   Distance away from Arc is less important than getting them behind and to the side of the main viewing area. 

Thanks. I will leave the surround speakers as they are then at ear level.

My surround speakers are set behind me and to visualise say if I am facing my TV (and Arc) at 12 o'clock then my left hand SL speaker is at 7 o'clock and my right hand SL is at 5 o'clock