Before you try to play audio, you'll want to free up some Arduino RAM, so that you don't end up with a nasty stack-overflow. Especially if you're running a Atmega168-based Arduino!
Note that the library is pretty big (about 10K) so if you want to do a lot more, I suggest upgrading to an ATmega328. The shield was designed with the expectation that this part would be available.
If you want a human voice in your project, you can use the free generator at AT&T Text-to-Speech demo page
It will create a 16KHz, 16-bit audio file so you can use the audio 'right out of the box'
Here is huge collection of C.C. Attribution licensed sound samples! A lot of it is already mono, 16 or 22KHz
This is the simplest example. It plays every audio file it finds on the SD card in a loop. This sketch is also included in the library
This example shows how to use the AT&T text-to-speech website to speak the first 2640 digits of pi. The number is stored in flash, each digit is spoken one at a time.
- Zip file containing the digits 0 thru 9 and 'point' place the wave files onto the SD card
This example plays 6 different files depending on whether buttons connected to the 6 analog input pins are pressed.
This one plays 4 different files depending on serial characters, good if you have say an xbee you want to use
By messing with the playback interrupt, you can change the speed of playback for an interesting effect.
Here is the sketch, connect the potentiometer to analog pin 0 (or change the code)