Actually, there is only one WAV file for the meeting (whereas each user has their own webcam file if shared). FreeSWITCH is writing the WAV file based on the audio it receives.
FreeSWITCH does give users the ability to increase/decrease volume using DTMF controls. In
opt/freeswitch/conf/autoload_configs/conference.conf.xml
you see the options
<caller-controls>
<group name="default">
<control action="mute" digits="0"/>
<control action="deaf mute" digits="*"/>
<control action="energy up" digits="9"/>
<control action="energy equ" digits="8"/>
<control action="energy dn" digits="7"/>
<control action="vol talk up" digits="3"/>
However, there isn't the ability for the user to send these DTMF commands (such as pressing '9' on your phone's keypad).
For now, we would recommend that users who are connecting via Flash audio (not WebRTC audio), make use of the echo test to hear how their audio sounds and use the built-in controls to increase audio gain.
Regards,... Fred