In the first 2 minutes of trying Pre-release 13w21a My sound just quit as soon as i jumped into a river to cross it and when i exited the game to see the log file i saw this.

Client> SoundSystem shutting down...
Client> Error in class 'SoundSystem'
Client> Command thread did not die!
Client> Ignoring errors... continuing clean-up.
Client> Error in class 'SoundSystem'
Client> Problem during Library.cleanup()!
Client> Ignoring errors... continuing clean-up.
Client> Author: Paul Lamb, www.paulscode.com