Is that 3.5mm port on the projector labeled as "Output"?
What is the exact model number of your projector?
You might have to use "HDMI Audio Extractor" device between the cable box and the projector, to feed audio signal to your audio system. Another (better) option is to connect your cable box' audio output directly to the audio system. Using optical cable (if available) will give you surround sound, everything else will be stereo only.
Edit: Your title reads "Computer", and your message says "cable box". What is what?