I've been using my RPi as an AirPlay endpoint for iTunes (using shairport) and found, like others, that the built-in audio didn't quite cut it. After messing with alsamixer up and down I decided to get a USB add-on sound card.
The first card I tried was http://www.newegg.com/Product/Product.a ... 6812186035 - It was an improvement over the built-in audio, however you could still hear static-y ticking while playing music.
Finally I found that using the included USB sound card from a Logitech G330 headset (http://www.logitech.com/en-us/webcam-co ... adset-g330) seemed to produce perfect sound quality. I picked one up on eBay for ~$13 USD.