http://www.html5rocks.com/en/tutorials/track/basics/
http://www.iandevlin.com/blog/2015/12/html5/webvtt-and-audio
It seems that captions/subtitles for HTML5 audio is actually notoriously badly implemented in most browsers right now.. Truly a shame that we will once again have to fallback to non-native behaviors if we will want to support this. :(