Record audio using HTML5 Microphone?

The standard versions of Chrome and Firefox fully support navigator.getUserMedia();

Numerous examples have been already made to record audio directly in the browser, in several formats even.

Here are a few examples:
This is a fully oriented project towards testing the limits and possibilities of the current state of webRTC
www.webrtc-experiment.com/RecordRTC/

I’ve personally made a record to mp3 library in JS sources can be found here:
github.com/nusofthq/Recordmp3js

Also for updates you can check the latest MediaStream Recording API proposal by the Media Capture Task Force. This is not yet implemented in the current browsers but it will in the near future.