JavaScript record audio webrts

Help me figure it out with JS AudioWebAPI. Using the library recorder.js I try to record telephone conversations (SIPML5 + asterisk). So far it has only sounded from the microphone:

var audio_context;
var recorder;

function startUserMedia(stream) {
    recorder = new Recorder(audio_context.createMediaStreamSource(stream));
    console.log('Recorder initialised.');
}

function startRecording(button) {
    recorder && recorder.record();
    console.log('Recording...');
}

function stopRecording(button) {
    recorder && recorder.stop();
    console.log('Stopped recording.');
    createDownloadLink();
    recorder.clear();
}

window.onload = function init() {
    try {
        // webkit shim
        window.AudioContext = window.AudioContext || window.webkitAudioContext;
        navigator.getUserMedia = navigator.getUserMedia || navigator.webkitGetUserMedia;
        window.URL = window.URL || window.webkitURL;

        audio_context = new AudioContext;
    } catch (e) {
        alert('No web audio support in this browser!');
    }

    navigator.getUserMedia({audio: true}, startUserMedia, function(e) {
        console.log('No live audio input: ' + e);
    });
};

How to record the sound of incoming stream? Tried to record the sound of the interlocutor in the following way - to no avail.

function startUserMedia(stream) {
    recorder = new Recorder(audio_context.createMediaStreamSource(document.getElementById('main_audio_remote').captureStream()));
    console.log('Recorder initialised.');
}

Thread webrts into tag 'main_audio_remote':

callform.oConfigCall = {
    audio_remote: document.getElementById('main_audio_remote'),
    screencast_window_id: 0x00000000, // entire desktop
    bandwidth: { audio: undefined, video: undefined },
    video_size: { minWidth: undefined, minHeight: undefined, maxWidth: undefined, maxHeight: undefined },
    events_listener: { events: '*', listener: callform.onSipEventSession },
    sip_caps: [
        { name: '+g.oma.sip-im' },
        { name: 'language', value: '\"en,fr\"' }
    ]
};