the
pbs channels you are refering to are using
ac3 audio and you will need a receiver that has ac3 audio pass through like a fortec lifetime star ultra. the signal will then need to be fed to a home theatre system that can accept ac3 also known as dolby digital to decode. my satwork 3688 was doing the same thing, perfect pic but no sound. i slaved my fortec on it and scanned the channels in and fed the pass through . perfect signal and audio.