Audio auto-normalization is ON by default in VDO.Ninja.
You can turn off auto-normalization by adding &autogain=0 to a source link (guest). If you are using &proaudio, auto-normalization gets turned off. You can enable it again with &proaudio&autogain=1.
You can also switch it on or off via the audio settings. If you want the guests to be able to change the audio settings by themselves, use &mediasettings on the guests' link, as per default only the director can change the audio settings of the guests.
Your browser will try to keep optimum audio levels.