You mean the small speaker for the ear, the loudspeaker or using headset?
We had similar problems with all of them regarding loudness already with Android 5.1 which also changed from update to update a little bit.
For me with Android 5.1 the small speaker was first ways to low while with headset it was way to loud. With an update also headset was way to low and not usable in street noise anymore. So I hoped the best, that with an update the overall dynamic (min/max) will be increased, but obviously not?!
https://forum.fairphone.com/t/minimum-volume-too-loud-when-using-headset/12913
https://forum.fairphone.com/t/low-volume-in-calls-with-speaker-mode/12610
https://forum.fairphone.com/t/fp-os-os-feature-requests-vote-for-your-favorites/12303/64?u=therob