On uC Hobby, they’re beginning a series on bringing sound to Arduino:

This is the first in a series of articles about generating sound with an Arduino. The goals are to generate good quality sound which can be used to play simple tones, stored music, sampled sound, and even act as a MIDI synthesizer. I will cover the basic design, including code and hardware that anyone can use to add sound to Arduino microcontroller projects.

The first step is to look what has already been done. I want to explore many possible options for sound generation and resolve which is best for given situations. I hope to spark some discussion so please comment with information links, advice, suggestions and general opinions.

Arduino Sound – Part 1 – Link