HTML5 `audio` tag controls disabled for WebRTC audio stream

In my project I have a WebRTC connection, whose audio stream I direct to the source element under the audio.

This is how I set it in code:

pc = new RTCPeerConnection(config);

// connect audio / video
pc.addEventListener('track', function (evt) {
    if (evt.track.kind === 'video')
        document.getElementById('video').srcObject = evt.streams[0];
    else
        document.getElementById('audio').srcObject = evt.streams[0];
});

And this is my HTML:

<audio controls>
    <source id="audio">
</audio>

The audio is playing, but the volume control is disabled.

I tried to control the volume myself, using the volume attribute but it didn't change anything…

Is there a way to control audio's volume when it is received from WebRTC audio stream?