Quote from Atmel application note AVR335:
This application note describes how to record, store and play back sound using any
AVR microcontroller with A/D converter, the AT45DB161B DataFlash memory and a
few extra components.
This application note shows in detail the usage of the A/D Converter for sound record-
ing, the Serial Peripheral Interface – SPI – for accessing the external DataFlash
memory and the Pulse Width Modulation – PWM – for playback. Typical applications
that would require one or more of these blocks are temperature loggers, telephone
answering machines, or digital voice recorders.