Would you care to elaborate as to what you mean by “sound is horrendous” with Era 300’s as surrounds. Many people in this community have Era 300’s as surrounds with both the Arc and Arc Ultra (present company included) and it’s extremely rare that the sound has been characterized as horrendous. In fact I can’t recall “horrendous” ever having been used to characterize the sound. 

What are you listening to:

• Music (stereo or Atmos)
• Movies (Stereo, Dolby Digital, Atmos)

Make sure you have actually added the Era 300s as surrounds and not just grouped them:

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

When I say horrendous, it is difficult to explain but I am watching movies or TV mainly. (I will do some tests with music tomorrow) 

it is almost as though there is a synchronisation issue and it is making sounds from the rears (Era 300) that just don’t sound right. 

And yes they were added as surrounds and not grouped

What model TV do you have? How exactly are you streaming TV and movies?

When I say horrendous, it is difficult to explain but I am watching movies or TV mainly. (I will do some tests with music tomorrow) 

it is almost as though there is a synchronisation issue and it is making sounds from the rears (Era 300) that just don’t sound right. 

Get it to sound bad, submit a diagnostic quickly and call Support when you can.

It shouldn’t sound bad, my new pair sounds fine.

I have the latest LG G4 with latest HDMI and have upgraded streaming services such as Netflix for best sound

Make sure the TV is set to these settings:

HDMI Input Audio Format: Bitstream

Digital Sound Output: Pass Through

eARC Support: On

If you want Dolby Atmos audio from Netflix, be sure you have the Premium subscription.

I have a ‘similar’ Sonos setup, except using a Sub (gen 3), rather than a Sub 4 and it sounds absolutely perfect here… so I agree with ​@Stanley_4, it’s probably best to reproduce the issue(s) and then immediately submit a Sonos system diagnostic report from within the Sonos App, note it’s reference and contact/chat with Sonos Support Staff via this LINK and see what the Staff can perhaps suggest to help you to resolve the issue.

I will check tomorrow but I believe it already uses eArc and bypasses everything as you can’t change any other settings. 

Thanks. I will do this and see what they come back with 

Keep in mind that surround speakers are not supposed to play the same audio that the Arc Ultra is playing. It only plays the audio data for the surround channels in the audio mix. You will mainly hear ambient sounds, the musical score, or sound effects that happen behind the camera.

You can do a sort of functional test by switching from TV mode to Streaming and then in settings toggle the Surrounds from the default Ambient to Full. 

When streaming a NON-TV source (Amazon, Spotify, Radio) and in Full mode they are basically a left/right pair carrying the full Stereo signal.

I don’t know of I am experiencing the same thing or not. My audio source is a Google TV just watching YouTube TV. 

With my head near one of my surrounds I notice that the sound from the 300 is “pulsing”, like someone is adjusting the volume turning it off then on again every half second.

Thanks. I do realise that and had a Yamaha amp and Monitor Audio surround set up prior to this. I never had issues like this. 

Okay I think I have resolved the issue. 
As the Arc Ultra is hard wired to the router I had disabled WiFi on it. Whilst the system appears to still work and you get sound etc from the rears, it does not work correctly and  from what I am reading, the Arc controls the other speakers via its own network so without WiFi enabled the rears must have been using the router WiFi and this caused the synchronisation issues. 

I think this is essentially correct.  I would query whether it is really a “network” that Sonos is using, but it is designed to use direct routing between the radios on the soundbar and surrounds. The wired soundbar will use the Ethernet connection whether the setting is for “Enable WiFi” or “Disable WiFi”,  “Disable WiFi” turns off the radio on the soundbar, making the intended direct wireless routing impossible.

Interesting. Mine is also hard wired. Will have to check to see if I have left “wifi enabled” or not.

One other thing I noticed is that I tried to do the wireless reset from the app, and it failed for all my devices.

Sorry for my ignorance, but I’m not sure what you mean by the “wireless reset”, and why you want to reset anything?

sorry, I think it was the “Update Network” in the help center. I thought I would try to do that to see if it was network related.

And just FYI - in order to disable wifi you need to have the sound bar wired to Ethernet (which makes sense). I may just play around with enabling/disabling the wifi on the sound bar to see if that has any impact on the sound pulsing I am getting from my Era 300 surrounds.

“update network” is for changing the WiFi credentials in Sonos, eg when changing router.  I can’t see that it has any bearing here.

I shall be very surprised if enabling / disabling WiFi has any bearing on your issue (if it is an issue).  Bottom line is: leave WiFi enabled on all devices.

What exactly is the problem you are trying to address?

Seriously, leave WiFi enabled.

If you are still considering it call Sonos Support and talk about it first.