I’ve just tried this on a Sonos Roam with Alexa and it’s around a second (or so) here after saying… “Alexa, turn up the volume” that the ducked volume restores to the higher setting.

Have you tried rebooting the affected speaker?

In addition to the settings you mentioned in your post, I have the ‘wake word chime’ set to ‘off’ …and ‘brief mode’ is also enabled in ‘Settings/Voice Responses’ in the Alexa App.

I’ve just also tried this again (a few times) on a stereo pair of ‘Ones (gen1)’ and those take just a second, or so, aswell, to restore their ducked audio. So my thoughts are that it might be worth trying a reboot of the local router/network too, perhaps? HTH

Have you also disabled and then enabled the Sonos skill in Alexa?

thanks Ken! 

Appreciate you doing a little testing on your end. Made me realize like it was a bug in my system not just my brain. 

I think switching off ‘chime’ was the trick as brief mode was already off.

Getting used to not having the chime will take some time but I’m happy with the result. thanks again!

@Tomagotchi,
I think I got to a stage in the past where the Alexa wake ’bong’ was beginning to grate on me. I don’t think I could go back to ever using it again - admittedly I thought it was going to be switching on ‘brief mode’ that was going to perhaps sort this for you (or a router reboot). Anyhow glad to hear it’s sorted.