Audio.js
A cross-browser javascript wrapper for the html5 audio tag
Midi.js
Making life easy to create a MIDI-app on the web. Includes a library to program synesthesia into your app for memory recognition or for creating trippy effects. Convert soundfonts for Guitar, Bass, Drums, ect. into code that can be read by the browser. Supports multiple simultaneous instruments and perfect timing.
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)
Timbre.js
JavaScript library for objective sound programming
Audiolet
A JavaScript library for real-time audio synthesis and composition from within the browser
Waud
Web Audio Library
Teoria
A JavaScript music theory framework.
Tone.js
A Web Audio framework for making interactive music in the browser.
Buzz
Buzz is a small but powerful Javascript library that allows you to easily take advantage of the new HTML5 audio element. It tries to degrade gracefully on non-modern browsers.
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.
Vexflow
A JavaScript library for rendering music notation and guitar tablature.