Sounds like a classic case of signal overload. The video signal going into the TV is too strong and is spilling into the audio. Does the cable buzz too or is it only the Xbox and DVD? Do you run the DVD and Xbox through a switchbox? How's the cable signal? Not knowing exactly how you're connected, I'd begin by recommending an inline attenuator like
this one to bring the signal down to a more tolerable level for the TV. If your DVD and Xbox run through a switcher and your cable is fine, put the attenuator on the cable going from switcher to TV and see if that doesn't solve the problem.