I can successfully add a gain node to an audio element sourced from a file, but when I try it with a stream / audio element derived from an RTCTrackEvent's audio stream it ignores the gain node. I was curious if this was possible at all.
There is a known bug in Chrome that causes the stream not to work unless you do a hack and include a muted audio element. See the answer by jmcker here for the method and a jsfiddle example. This solved my issues.