It's not in the WebRTC API. You can change the volume on the audio tags you create, or you can pipe the audio media stream into a WebAudio graph and modify it there.

I've built a number of WebRTC apps over the years. Recently, I built just such a thing as you described and open sourced it: https://www.calla.chat. I opted to build it on top of Jitsi Meet this time. It's actually advantageous that it's not through the WebRTC API because Jitsi doesn't give access to the raw WebRTC commands. But hijacking the audio elements it creates is completely doable.