Hi ​@Cdtate, welcome to the Sonos Community!

I assume you mean the Era 100, as we don’t have a speaker called the Arc 100. You can adjust surround sound settings, such as volume, in the Surround Audio menu in the Sonos app. You can find this by going to Settings → Selecting your home theater setup → Surround Audio.

You can’t adjust the volume of an individual surround however as any setting changes will adjust both surround speakers. If it’s just the one speaker having issues, I’d suggest removing the surround speakers, testing if the speaker you’re having issues with is too quiet when playing by itself, and if it isn’t, add the speakers back as surrounds.

If you notice that the speaker still has volume issues when it’s not paired in the home theater configuration, then I’d suggest contacting our support team for further troubleshooting.

I hope this helps!