This app is the beginnings of a DJ deck built in Web Audio. It started from a discussion I had with Scott Schiller, where he asked "hey, can you set playbackRate on a BufferSourceNode to a negative value?" (The answer to that, by the way, is no.) It's grown quite a bit from there, especially after I purchased a Numark DJ-2-Go controller and plugged it in through the Web MIDI API polyfill. It's much cooler with the controller; you can't currently scrub from the onscreen UI. (Yes, I'll get to that.) Note you can drag and drop new sound files into the decks.
Check it out, feel free to fork, submit pull requests, etc.
-Chris