audioconverter.js is a Javascript port of FFmpeg with audio support for the most common audio formats. It allows user to transcode audio files directly within a browser with almost native speed without uploading any files to a remote server.
File formats supported: wav, mp3, ogg, aac, wmv
External libraries ported and included: lame, ogg, vorbis, fdk-aac
Check out quick-apps.com/audio to see how you can transcode audio files directly within a browser without installing any additional plugins.
To build go to ffmpeg.js directory and run (assuming that you have emscripten configured): ./emcc-audio-build.sh
Check out my blog about audioconvert.js.
Another cool FFMPG Javascript project: https://github.com/bgrins/videoconverter.js