I’ve made an audio recorder that records to mp3 directly from the browser combining RecorderJS and libmp3lame.js
You can find the gitHub project here:
https://github.com/nusofthq/Recordmp3js
and a more detailed explanation of the implementation:
http://nusofthq.com/blog/recording-mp3-using-only-html5-and-javascript-recordmp3-js/