Despite the name, it is unrelated to the bidirectional I²C bus. I²S is a media-specific protocol which uses at least three signals: Bit CLK, Word CLK, and at least one data line.

Inter-IC Sound (I²S) is a serial bus interface standard used for connecting digital audio devices. The I²S bus separates clock and data signals, resulting in a very low-jitter connection. The bus consists of three lines: a clock line, a word-select line, and a multiplexed-data line.

Wikipedia article for I²S
I²S Bus Specification (archived by the Wayback Machine on July 2, 2006)