Sorry, I initially misread this post. It is a white PCE. Got it.
If the RF cable isn't bad, which I'm guessing its not, the two audio caps that control the sound are probably leaking or on their way out.
There are a few people here on this forum that can help you with the repair if you can't replace the capacitors yourself. I'm not taking repair work at the moment, but would suggest contacting chop5, BlueBMW or thesteve. I think Keith Courage also does repairs.