https://developer.mozilla.org/en-US/docs/Web/API/AudioTrackList/addtrack_event
AudioTrackList: addtrack event - Web APIs | MDN
The addtrack event is fired when a track is added to an AudioTrackList.
event webaudiotracklistapismdn
https://lists.w3.org/Archives/Public/public-html-bugzilla/2011Nov/0170.html
[Bug 13358] video also fire a 'change' event at VideoTrackList, AudioTrackList, and TextTrackList...