As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING FLIGHT 737 A LOUD BUZZING SOUND IN THE HEADSETS AND SPEAKERS LASTING A COUPLE OF MINUTES. DURING THIS TIME SMELLED HOT ELECTRICAL SMELL. VOLTMETER INDICATING 24 VOLTS. COULD NOT DUPLICATE. CHECKED SPEAKERS OUT, SPEAKERS OKAY. CHECKED AVIONICS OUT IN COAT CLOSET, ALL OKAY. RAN SYSTEM FOR 3 HOURS WITH NO PROBLEMS.