I’m not sure if this is an Amazon issue, or Sonos issue, but the volume of Alexa is intrinsically linked to the speaker audio volume and can’t be altered as far as I’m aware. I’ve seen folk here ask for Alexa to be both ‘quieter’ and ‘louder’ and presumably what we are hearing now, is an Alexa audio volume curve based on the majority of customer feedback, but it’s perhaps a case of not ever being able to please everyone.
 

I personally suspect Amazon may have the final say in some of the things here and I agree there ought to be some kind of slider control/adjuster to alter the curve in some helpful way… it’s something I’ve seen discussed in many threads, so I’m pretty certain that Sonos engineers are aware of this matter.
 

Sonos rarely, if ever, announce their development roadmap, so only time will tell if anything will change in this area.