I2S protocol interfacing with an STM32 microcontroller app note from ST Microelectronics. Link here (PDF)
The I2S protocol is widely used to transfer audio data from a microcontroller/DSP to an audio codec in order to play melodies (stored in a memory) or, to capture analog sound (from a microphone).
The STM32L1xx allows I2S audio communication using the SPI peripheral, and implements specific functionalities for this communications mode.