I'm actually surprised that I got a response from United's customer service department.
What's interesting is that United appeared more concerned about the malfunctioning sound system than the overcharge. United's customer service rep started off his email with:
"We realize that any type of malfunction can impact your perception of our overall operation. Please know that we place a great deal of emphasis on proper maintenance of all equipment in our cabins. In fact, our maintenance division has recently completed an audit of each fleet type and taken inventory of problems. Based on that we have established a program to check the cabins of each aircraft every 400 to 1,000 flight hours. We will be identifying and repairing seats, carpets, galley equipment, lights and audio and video equipment. In addition, another maintenance team is concentrating on cleaning methods, audio and video functionality and improved cabin condition monitoring. Still, your disappointment with your flight is understandable, and I will give your constructive feedback to our maintenance and onboard management because we are committed to providing you a positive in-flight experience."
As a good will gesture, United gave me a $100 travel voucher for future travel. The customer service rep also indicated that United will be issuing me a check for $9 to reimburse me for the overcharge. Hurray!
I cautioned that we should all check our receipts at the time of purchase. But it always helps to voice our complaints. If the company values your business, they will do you right. Even if it's an airline.