Control volume gain for Video/Audio stream in Firefox - firefox

Question

I'm trying to record Video/Audio files using MediaRecorder API for Firefox.
When I'm using web Audio API for creating the nodes (source -> Gain -> Destination)
The output of the recorded file is only Audio as the return stream from the destination node is Audio stream only referring to this documentation
https://developer.mozilla.org/en-US/docs/Web/API/MediaStreamAudioDestinationNode
Any suggestion for getting Audio/video stream in the destination output to record audio/video not audio only.
var mediastream;
var ctx = new AudioContext();
var mediaStreamSource = ctx.createMediaStreamSource(mediaStream);
var destination = ctx.createMediaStreamDestination();
ObjectStore.VolumeGainNode = ctx.createGain();
ObjectStore.VolumeGainNode.gain.value = 0.5;
mediaStreamSource.connect(ObjectStore.VolumeGainNode);
ObjectStore.VolumeGainNode.connect(destination);
mediaStream = destination.stream;

Answer 1

You need a stream composed of the gUM video track and your gain-modified audio track.
Following the standard, Firefox lets you modify tracks in a stream using stream.addTrack and stream.removeTrack, as well as compose new streams out of tracks with new MediaStream([tracks]).
This lets you solve your problem by replacing the gUM audio track with your gain-manipulated one:
var constraints = { video: true, audio: true };
var start = () => navigator.mediaDevices.getUserMedia(constraints)
.then(stream => modifyGain(video.srcObject = stream, 0.5))
.catch(e => console.error(e));
var modifyGain = (stream, gainValue) => {
var audioTrack = stream.getAudioTracks()[0];
var ctx = new AudioContext();
var src = ctx.createMediaStreamSource(new MediaStream([audioTrack]));
var dst = ctx.createMediaStreamDestination();
var gainNode = ctx.createGain();
gainNode.gain.value = gainValue;
[src, gainNode, dst].reduce((a, b) => a && a.connect(b));
stream.removeTrack(audioTrack);
stream.addTrack(dst.stream.getAudioTracks()[0]);
};
Here's the fiddle (Firefox 44 or newer): https://jsfiddle.net/7wd2z8rz/
Again with MediaRecorder: https://jsfiddle.net/j33xmkcq/