Era300 surrounds modulate the volume

Certainly seems odd, I have not. 

I would recommend that you submit a system diagnostic within 10 minutes of experiencing this problem, and call Sonos Support to discuss it. Don’t post the resulting diagnostic number here, their lawyers get sensitive about GDPR.

There may be information included in the diagnostic that will help Sonos pinpoint the issue and help you find a solution.

When you speak directly to the Support staff, they have tools at their disposal that will allow them to give you advice specific to your network and Sonos system.

Same setup here (with old Arc) and never noticed something like that. 
It’s true that, as surrounds, they never play sounds really loud. Perhaps in action scenes but then everything sounds  high and I never feel an up and down from rear speakers. 

Are you actually grouping a stereo pair of 300s with the Arc where the Era 300s are a different room, or are you pairing the 300s as surrounds with the Arc? There is a difference between the two.

Grouping is easy to do from the Arcs room page. Tap the speaker icon with the “1” and select the other room. The same audio will play in both rooms.

Pairing is when you add the Era 300s that you have not setup as a stereo pair to the Arc one at a time as surrounds They will appear in the app as two different rooms. It is not easy to switch the speakers to play independently. They must be removed as surrounds and set up again as different rooms or as a stereo pair.

Which of these scenarios refers to what you are asking about?

As surrounds, they will only play the audio that is sent to them from movies and shows. If there is no surround audio, there will be no sound from them.

 Music is different because there is a setting to have them play full audio instead of ambient audio.

If the issue you are experiencing happens to both music and film, I suggest you give Sonos support a call and submit a diagnostics. They should be able to tell you if it is a speaker issue or a network issue.

This isn’t normal behavior. When Era 300 surrounds “pulse” or modulate volume while grouped with an Arc Ultra, it usually points to a wireless interference or sync issue between the Arc and surrounds.

Try these steps:

1. Reboot the Arc Ultra + both Era 300s + router.

2. Make sure your Arc Ultra is on wired or 5GHz, not 2.4GHz.

3. Move any nearby Wi-Fi devices or smart hubs away from the Arc (they can disrupt the Sonos wireless link).

4. Remove the surrounds from the Arc → re-add them as surrounds instead of grouping.

If the modulation disappears when used as true surrounds after re-adding them, then it was a pairing/sync issue. If it persists, Sonos Support can pull diagnostics — this behavior usually indicates interference on the dedicated surround wireless channel.

Technically, Sonos doesn’t ‘pull’ diagnostics, you have to submit them, as indicated in my response. They then ‘pull’ the results from the server where your system submits them to. 

Thank you all for your replies.

The Era 300s are set up using the App as surrounds connected to the arc ultra and are in the same room either side of the sofa positioned as you would expect surrounds to be. When a Dolby Atmos source such as Netflix is playing the surrounds are indeed fairly quiet, as they are when a music source is playing. In this scenario, the sound modulates whether the source is Netflix or a music source. If I use the app to disconnect the surrounds then I play any source with the arc Ultra and the surrounds in a group, then there is no modulation and everything works fine. It's only when the surrounds are linked to the arc Ultra in the app that I have an issue. Thanks again for your comments.

From what you just described IMO indicates an issue in the hidden 5Ghz frequency between the Arc Ultra and the Era 300’s. Reboots obviously have no effect. I know what I would do and that is to factory reset all components Arc Ultra and Surrounds and go through the setup process again. Generally I’d not recommend a factory reset. However, if after doing so and setting things up again and the issue returns you can still run a diagnostic and call Sonos tech support. If you don’t feel comfortable doing the above then just run the diagnostic and contact Sonos support as previously suggested. Good luck!

This disconnecting the Eras is something you did just for testing, right? Just to be sure that you’re using the rear speakers in a real surround (as ​@Pools-3015 explained above) and not just grouping them to the Arc in the same room (understanding room as the group shown in the app), where during TV streaming it should sound with problems. Do you perform TruePlay again when you installed the two rear as surrounds?

Another issue I suffered when I wired the Arc is to switched off the WiFi setting, something you shouldn’t do because then Sub and rears start to work wrong. 

PS

I forgot to say that the arc ultra is wired and the Era 300's are wireless. I have power cycled all 3 units as well, with no resolution.

This disconnecting the Eras is something you did just for testing, right? Just to be sure that you’re using the rear speakers in a real surround (as ​@Pools-3015 explained above) and not just grouping them to the Arc in the same room (understanding room as the group shown in the app), where during TV streaming it should sound with problems. Do you perform TruePlay again when you installed the two rear as surrounds?

Another issue I suffered when I wired the Arc is to switched off the WiFi setting, something you shouldn’t do because then Sub and rears start to work wrong. 

Yes I disconnected just for testing, the surrounds were paired initially. I haven't performed TruPlay again, but will try all suggestions above and report back.