The Vstamp from RCsys.com is a bit pricey... Not bad if you want a nice voice and unlimited vocabulary.  I am also laying out a cheaper alternative.  It will fit in the same spot as the Vstamp and be called the WAVstamp.  It wil lhave an uSD card and a 1.4W amp.  This will require a bit more I/O, code space and memory (4 IO for the SD card and Pin 9 PCM for the WAV output).  With the WAVstamp, you can put WAV files on the uSD card and play them back using the TMRpcm library through pin 9 that will be wired to the AMP so the audio output is amplified to an 8ohm speaker.