Running 14.0.10 (141024). Problem started about a month ago. May be coordinated with corporate upgrade to Skype for Business, which I recently became aware is being rolled out.
During a group call, volume constantly increases to maximum, requiring constant adjustment back to original volume. Very frustrating!
I'm not positive, but this seems to be related to either people joining or leaving the call, or possibly someone else adjusting their volume. Sometimes this happens every 20 to 30 seconds, sometimes it stops for many minutes. Tends to happen a lot near start of call or end, which is why I suspect it's people leaving or entering or adjusting volume.
AFAIK, running most recent version of Mac Lync client, upgraded relatively recently, in 2015.