What was found
WITH NO PASSENGERS THE PILOTS EXPERIENCED A LOUD SQUEALING IN THEIR HEADSETS SO THEY TURNED OFF ALL ENVIRONMENTALS & THE NOISE WENT AWAY. THEY TURNED AROUND & WERE COMING BACK TO DEPARTURE, WHEN THEY SMELLED WHAT THEY THOUGHT WAS SMOKE. THEY DECLARED AN EMERGENCY & LANDED UNEVENTFULLY. THE AUDIO AMPLIFIER WAS THE CAUSE OF THE NOISE & THE BLEED AIR DUCKS WERE THE CAUSE OF THE SMELL, WHEN THEY GOT SO HOT FROM THE BLEED AIR BEING ON WITH NO BLOWERS RUNNING.