This could be easy emulated by single channel PWM or dual channel PWM signal (>50Ohm speaker with small enclosure could be connected directly); from internal counter.
Bare microphone should be amplified to get a minimum 1Vpp analog signal. There are some MEMS microphones with SPI output, so ADC will not be necessary.
By the way. Build in analog converter has also limited speed. So max. frequency response is low. IMO worse than AM radio.