Era 100

Sounds a lot like you have them grouped , rather than set up as surround speakers for your Arc. If that’s the case, they’d be in a separate ‘room’ and not affected by changes to the volume using the remote to control your Arc. 

I’d suggest adding them to the Arc’s room as surrounds, you may find that to be much more useful to your setup. 

FWIW, any and all WiFi based speakers are subject to wifi interference , even that which is generated outside your home. I had issues at one point when a new neighbor moved in next door, and set up their WiFi on a channel that stomped all over mine. Not to mention the time my microwave started blasting interference, but that was internal to my home. But either way, I had to identify the source of the issue, and rectify it. 

As a reminder, since they’re not currently set up as surrounds, they’ll be connecting directly to your WiFi’s signal, whether it is 2.4 GHz or 5 GHz. When they’re set up as surrounds, they’re switched over to 5Ghz only, and connect to your Arc directly, so if there’s still interference,  you can narrow it down some (most often, TVs can ‘leak’ in the 5Ghz range, but it isn’t guaranteed).

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.

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.

Yeah, if you have the Era 100s in the same room as the Arc and they are separate rooms in the Sonos app, then you aren’t making the best use of them.  If you intended for a surround system, you need to add the Era 100’s to the Arc as surrounds:

To add as surrounds:

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

(Note the link to separate a stereo pair before adding as surrounds)

This will “bond” the Era-100s as surround speakers, linking them with the Arc as one room (Left-Center-Right-Left-Right Height in Arc, Left-Right Surrounds in Era-100s).