Based on the discussion here: https://groups.google.com/a/chromium.org/d/topic/blink-dev/Yk8u329AmIs/discussion

Specifically "it look like "audio_track0" ended up in the name field of a user data box ('udta'). So maybe https://dev.w3.org/html5/html-sourcing-inband-tracks/ should use that instead. Needs investigation and discussion."

Seems like we need to expose a different field from the media stream to the page via the label field.