Ability to boost a participant's volume

### Your use case

#### What would you like to do?

Boost the volumes of particular participants beyond their default value

#### Why would you like to do it?

Because sometimes people have unusually quiet audio. The alternative is to turn everyone else's volume down, and then turn up the system volume, but this is tedious.

#### How would you like to achieve it?

The knob on the volume slider should sit somewhere in the middle by default, so it has some headroom to be turned up.

On the code side, if a participant's volume has been turned up, we'd need to use the Web Audio API to increase the gain.

### Have you considered any alternatives?

> The alternative is to turn everyone else's volume down, and then turn up the system volume, but this is tedious.

### Additional context

_No response_