The second connector is the rear channel connector. The HK 695 speakers have two input connectors. The green connector is the front channel connector. If your sound card has only one line out jack, the second connector will likely never be used. The second connector is the rear channel connector.
Master volume controls for the HK-695 speakers are located on the top of the right satellite speaker. NOTE: The satellite speaker that plugs into the back of the subwoofer with the square orange RCA connector is the right speaker.