Flac.js
FLAC decoder in JavaScript
Timbre.js
JavaScript library for objective sound programming
Amplitudejs
A Javascript library that allows you to control the design of your media controls in your webpage -- not the browser. No dependencies (jQuery not required)
Wad
Web Audio DAW. Use the HTML5 Web Audio API for dynamic sound synthesis. It's like jQuery for your ears.
Sound JS
A Javascript library for working with Audio. Features a simple interface as the front end to multiple audio APIs via a plugin model. Currently supports HTML5 Audio & Flash.
Pizzicato
Library to simplify the way you create and manipulate sounds with the Web Audio API.
Tone.js
A Web Audio framework for making interactive music in the browser.
Sound Manager 2
A JavaScript Sound API supporting MP3, MPEG4 and HTML5 audio + (experimental) RTMP, providing reliable cross-browser/platform audio control in as little as 10 KB. BSD licensed.
Speak.js
Text-to-Speech in JavaScript using eSpeak
Alpha Tab
alphaTab is a cross platform music notation and guitar tablature rendering library.
Teoria
A JavaScript music theory framework.