DTMF have been used to transfer data through sound, however it limits the data rates. In this project they have used a speaker and a microphone system to transmit the data generated. The data is generated using a PS/2 remote which is then processed by the microcontroller and the audio signal is fed to a speaker. The sound is detected by the microphone attached on the user side which is then processed and displayed on the LCD attached to the receiver side.
The TX-MCU samples DATA on the falling edge of CLK to build a data packet representing the key that was pressed, then transmits the packet as a combination of two alternating on/off tones (one at 8 kHz, another at 5 kHz) over the speakers, where one tone represents CLK and the other represents DATA. The current design uses a very simple protocol called On-Off Keying on two tones. Upgrading the code to use a more sophisticated protocol would increase the data rate. The change in speaker and microphone specifications might also increase the range of the system.