My setup: ARC + 2 x oneSL (surround.) My ARC is defined as a Speaker in Alexa. It is not in the same group as the Echo.
When watching TV, if I say ‘Alexa turn on lights’ - it does - and provides audio feedback via Echo speaker. BUT, then the sound coming from ARC goes and stays very low for ~15 seconds before returning to normal. If I say ‘Alexa stop’ during this period, the volume pops right back up. This seems to happen if the music is coming from the Echo as well (though, in this case, saying ‘echo stop’ actually stops the music too.)
My Echo is located behind my head, close to me, and it is the one that answers - not the Sonos speaker, when I talk to it. Alexa audio playback happens, as desired, on the Sonos.
Thanks for any and all assistance in fixing this uniquely annoying issue.