A Host or Producer can adjust a participant's output volume in the Studio sidebar to balance the sound during the live call, but this does not change the level of the recorded audio signal.
If a participant's computer has its microphone sensitivity too low or too high, they should adjust the settings on their device so their voice will not sound distorted on the final recorded track.
Although a participant cannot change the mic gain level on their mobile device, they can adjust their settings:
The volume meter in the Studio screen's People sidebar indicates each participant's audio level. When a participant speaks, if the meter's maximum level is consistently peaking on the right (high) end of the meter or only at the left (low) end, then they should adjust their device's settings or reposition their mic.