This is a demo page for a audio HTML web component, which wraps a standard responsive audio element and makes it respond to media changes that happen after pageload (which it would not do by default), while syncing the audios's timecode as well.
The web component is only bound after a feature test that makes sure the browser does not support reassessing audio source media changes after page load. This is just extra cautious, as at time of writing, no browsers do that and there's no known plan for that to change.