Your Home Theater System (HTS) does not have any type of video input, only audio. The yellow composite RCA jack is for video output only.
Check to see if your HDTV has an analog (red/white RCA) output. If it does, then you should be able to connect the HDTV's Red/White audio output to the HTS's Aux input. Your cable box would then be connected to your HDTV and the HDTV should send audio to the HTS.
If your HDTV only has an optical audio output, then you could try using an digital optical to analog converter (
something like this).
-Wolf sends