Sound functionality can push your design to new level. It may seem that playing recorded sounds with micros like AVR is quite intense, but in reality this is easy. And the only limitation here is memory storage. Husham Samir published his instructable on how to play wav files with Arduino Nano 3.0. His set up uses SD card for storing wav files striped to to 8-bit and 16000Hz sampling. This indeed is enough to get decent quality of voice messages.
There are four buttons used that allow selecting one of four files to be played. Player code relies on TRMpcm library which is specially created to play PCM/WAV files directly from SD card.