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.
Wad
Web Audio DAW. Use the HTML5 Web Audio API for dynamic sound synthesis. It's like jQuery for your ears.
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)
Waud
Web Audio Library
Audiolet
A JavaScript library for real-time audio synthesis and composition from within the browser
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.
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.