Can I only share audio from a webpage?
When I'm in a conference, can I share the audio from another browser window (youtube) to share just the music and mix it to my microphone?
You can capture audio stream from canvas as described here, then publish audio only stream as recommended above.
But it seems much simpler to publish audio only stream from virtual webcam or using ffmpeg.
You can use stream mixer functions at server side if you want to mix a different streams. Please read also here if you wish to dub a video stream with some audio one.