1. Try the internal tuner directly from your antenna through the internal speakers, then your sound system,
2. If ok, how about another device on another input? DVD player? To internal audio, then your sound system. You stated this was ok?
3. Have you run the audio directly into your sound system from your STB?
4. Narrow it down to one item.
Abundant OTA television is what makes this country different from all others. Lets keep it this way. If you like Wi-Fi so much, OTA fits right in. After all, it is wireless.