It's no secret that you can play various digital audio files with an Arduino, however most solutions require either a decoder IC and various circuitry which generally fills a protoshield and can cost more than thirty dollars. However if you just need to blurt out .wav files without too much ceremony there's a much simpler and cheaper method of doing so, demonstrated by Maksim Surguy:
In his method one simply needs an inexpensive SD card socket, an NPN transistor and speaker for output, as well as the Arduino of course. From a software perspective you make use of the TMRpcm library which allows for direct playback of PCM-encoded .wav files from an SD card. And that's it - you can gather the required hardware for less than $10.