Aiwa TV model TV-F2400U weird audio problem

Hello all- This is a fairly new set (2000 model). Picture is great.
But some channels have no audio at all even though the picture on
these channels is very good. I noticed that when the signal is weak
audio is muted. But this customer has cable service so I assume
signal strength is good on those channels since the picture is nice
and clear. I tried auto programming without success. Can anyone
explain that ? Thanks