Change the Volume Limit at System Settings > <select your product> > Volume Limit

I have set it at 40% for my stereo pair of Sonos One speakers and usually have volume at 4 at night time.

https://support.sonos.com/en/article/setting-a-volume-limit-on-sonos-products

“Note that the Now Playing screen will still show a full volume slider but will adjust to the limit you set. If you set a product's max volume to 50%, sliding the volume bar all the way to the right will output a volume of 50% instead of 100%.

Any volume limit you set will apply to line-in autoplay as well as any alarms you’ve set.”

https://www.sonos.com/en-us/guides/sonosapp

Sadly, I think that the volume limit has no impact on the lowest volume achievable. 

To the best of my knowledge there is no way to reduce the lowest volume.

@kpr . But 4 with a 40% volume limit  is actually louder than 1 with no volume limit.

You will find that with a limit of 40% there is no sound at all at positions 1 and 2. The sound is only there from position 3, which is already louder than at 1 without a limit.

You gain precision of control but not a lower minimum volume, so this does not really meet @KoenF 's need.

You are right. Didn’t realise and read properly. Is the only option then to try disabling loudness if that would lower the sounds at least a bit.

@KoenF which speaker are you using? I will likely run into the same problem of too high a volume if it’s a newer Sonos device?

I played with that setting, decreasing it and increasing it and then adjusting the volume, and on my Sonos One stereo pair there were these kind of issues. I did wait a bit between adjusting in order for it to take effect: 

• the queue got suddenly wiped empty on one occasion
• volume slider on Now playing screen lost the mute symbol on the left and number on the right 
• the left speaker lost all sound, right still played
• overall volume changes were sluggish

Since about a week the left speaker of the stereo pair has suddenly been silent on multiple occasion. I’ve needed to reboot it and then it works again. I wonder if it’s related to adjusting volume or just some issue that some other users seem to be having with Sonos One or stereo pairs.

Edit: never have had this kind of issue of the left speaker losing sound before. It still shows as connected stereo pair when it’s silent and right one plays. The system is on Wi-Fi. Could of course be that the around 4-year-old One is getting defective for some reason.

why are you doing

@kpr . I think it's very unlikely that there is anything wrong with your speakers.  It sounds like classic network latency. 

First thing to try is power off router and Sonos speakers,  power on router, power on Sonos. 

@kpr  I'm sorry  This is not the solution, volume stays high

I have 2 play-one in the office.

Thank you for the info. Sorry that didn’t indeed solve it. Those seem older devices. Has the volume always been that high or have the recent firmware/app updates/something changed it?