samsungson,
I was asking about how the DVD player was hooked up because I was trying to troubleshoot the source of the hiss. At the moment, the source of the hiss could be the DVD player, the TV, or the boombox. Yeah, I'm counting out the cables because I honestly don't think that they have anything to do with this.
Here's some things that I would try (be sure to turn things off before connecting/disconnecting cables):
1. Disconnect the input cable from the boombox, set the input selector to that input (so, the same that you've been using to listen to TV and DVD), and turn up the volume to the point that you normally hear a hiss. If you hear the hiss, then it's the boombox (either the main unit or the speakers) because there's nothing plugged into it.
2. If you don't hear any hiss, plug the cable back in and turn up the volume on the boombox without the TV being on. If you hear a hiss, well, I'll have to think some more about what's causing it, so let us know if that's the case.
3. If you still haven't heard a hiss, connect the patch cable directly to the DVD player (i.e. don't route it through the TV). Play a DVD and turn up the volume on the boombox. If you hear a hiss now, then it's the DVD player.
4. Still no hiss? Well, my friend, the issue has been narrowed down to your TV.
You are using your TV as a signal router, and depending on which audio output that you are using, you are also using it as an amplifier. I wouldn't be surprised if it's the source. I also wouldn't be surprised if the boombox is the cause, as others have mentioned already.
Well, good luck, and please let us know what you find out.
Adam