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Question: What am I doing wrong?

Hi, got a new theatre system for Xmas. Its a Coby (CS-P63), with a DVD-755, the TV is a magnasonic. The dvd player plays music cd's, the FL/FR work as they should for the music ( I used these plugs to check all the speakers to make sure they worked), and they do, so they are wired right. However, the sub has a low hum, and when I put in a dvd I only get sound from the TV. I think there is some sound from the centre speaker, but no dialogue. i triple checked all the wiring for the system, and tried all the options in the manual, but have gotten no results. I tried to contact the company but of course got machine after machine and no humans. I sent off an email but expect no answer. I also tried to adjust the audio on the setup in the dvd player but only have the options of changing the room type, and that's it. the volume on the Sub doesn't seem to give any results either. What am I doing wrong?