MediaStreamAudioSourceNode.mediaStream

The MediaStreamAudioSourceNode interface's read-only mediaStream property indicates the MediaStream that contains the audio track from which the node is receiving audio.

This stream was specified when the node was first created, either using the MediaStreamAudioSourceNode() constructor or the AudioContext.createMediaStreamSource() method.

audioSourceStream = mediaStreamAudioSourceNode.mediaStream;

A MediaStream representing the stream which contains the MediaStreamTrack serving as the source of audio for the node.

The user agent uses the first audio track it finds on the specified stream as the audio source for this node. However, there is no way to be certain which track that will be on multi-track streams. If the specific track matters to you, or you need to have access to the track itself, you should use a MediaStreamTrackAudioSourceNode instead.

const audioCtx = new window.AudioContext();
let options = {
  mediaStream : stream
}

let source = new MediaStreamAudioSourceNode(audioCtx, options);
console.log(source.mediaStream);