Some basic techniques for following audio with Tone.js.
Tone.Analyser does FFT as well as waveform representation.
https://jsfiddle.net/yotammann/noecnjeq/12/
https://jsfiddle.net/yotammann/97yw5q6m/11/
https://jsfiddle.net/yotammann/thz9kame/2/
https://jsfiddle.net/yotammann/95phq4fe/6/