Wave.js : Audio visualizer library for javascript. Create dynamic animations that react to an audio file or audio stream.
Timeline : Beautifully crafted timelines that are easy, and intuitive to use.
jsfx : This is a javascript library (with frontend) for sound effect generation.
jsmad : jsmad is a pure javascript MP3 decoder, based on libmad, with an ID3 decoder written from scratch.
SoundManager 2 : Using HTML5 and Flash, SoundManager 2 provides reliable cross-platform audio under a single JavaScript API.
dancer.js is a high-level audio API, usable with both Mozilla's Audio Data API and Webkit's Web Audio API with flash fallback, designed to make sweet visualizations.