If you still want to go ahead, you need the following:
Mic + amplifier
ADC (there might be one built in to your AVR)
Some kind of storage - you'll need about 8000 samples per second.
DAC (you can easily build an R-2R ladder for starters).
Storage can be static ram (but you might find it tricky to interface, you'll need something like 26 spare pins) or serially accessed flash ram. You'll need to be able to store and retrieve a sample with a cycle time of around 100 microseconds, and you'll need about 8k bytes for each second of speech.