You connect external speakers to the computer, not the projector. The extermal speakers must be powered, or amplified speakers because the computer does not have the power to run non-amplified speakers.
Connect the InFocus audio cable to the audio out port on the computer (usually marked with a picture of headphones). Then connect the other end of the cable to a pair of external amplified speakers. Plug the speakers into a power source, and the system will produce excellent sound capable of filling large rooms.