My sennheiser set 840 tv stethoscope listening headphones audio stops working after a few seconds. It sits right near the Tv and worked fine for years. The sennheiser unit plugs right into my cable box and those connections had not been touched for years. I replaced the cables from the unit to the cable box. I replaced the headset battery and the power source is feet away. The headphones still blink when turned on, the sound is great, and then -- nothing. What else could be the problem?