For anyone that wants to add sound playback capability to an MCU project you cannot find a simpler solution than the SOMO module that I linked to at SparkFun in my first posting. The module is a small postage sized PC board with stake pins for mounting it. It has a socket for a micro SD card and a small controller chip that is able to read ADPCM encoded sound files off the card and play then through a codec on board the chip. The module can directly drive a low wattage speaker or can be connected to an amplifier such as an LM386. In my case I connected the speaker output to a small transformer that converted from 8 ohms to 1K ohms to drive into the LINE input of a large high powered guitar amp. (We had to have loud sound for the monster roar which you can hear in the video if you turn up the volume).

You encode the files from a PC into the ADPCM format and transfer them to the microSD card as files named "0000.AR4", "0001.AR4", "0002.AR4" up to "0511.AR4". A low speed two wire serial interface, similar to SPI but output only, is bitbanged from the MCU with 16-bit commands to select the sound bite file to play.

Michael Karas