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Sound quality poor with Comcast cable box and Sylvania Plasm

My mom-in-law just purchased a new Sylvania 6842 PF Plasma monitor. She has Comcast digital cable with a Motorolla DCT2000 Cable box.
When the TV is hooked up directly to a DVD player or VCR without the cable box hooked up at all the picture/sound is perfect.

Once the cable box is hooked up directly to the TV or to the TV through the DVD player there is a loud humming in the background of the sound and also a horizontal line that slowly scrolls from the bottom up.

Connects are with a brand new set of RCA cables. Tried S video for the video signal and same problem occurs with the scrolling horiz line.

Can anybody tell me if there is a compatibility issue with the box and TV? Is this electrical interference that can be solved with some type of filter?
I don't have Comcast cable, will they upgrade the box for no charge if this is a compatibility issue between the box and TV?

Thanks in advance for any assistance.

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