I am trying to capture the microphone and send the recording to my server.. I tried this method here but it records only a big WAV and the upload can be slow sometimes.
You can compress the file on the clientside using libmp3lame.js: https://github.com/akrennmair/libmp3lame-js
There is already a gitHub project that uses this library to record audio and save it in MP3 format directly in the browser:
https://github.com/nusofthq/Recordmp3js