Have you ever fallen asleep watching television only to be rudely awoken by one of those obnoxious commercials that randomly play three times as loud as everything else? While the FCC actually made excessively loud commercials illegal as of last December, it's up to providers to comply, so the problem will probably take a while to go away.
Instructables user techbitar (Hazim Bitar) decided to take matters into his own hands by building an Arduino-controlled device that automatically adjusts your TV's volume so that it never goes above a certain level.
His TV Volume Loudness Guard is composed of an Arduino, breadboard, sound sensor, infrared transmitter and receiver, and resistor, and uses the remote control protocol for your TV to simulate pressing the volume down button when it senses that it's gone over the threshold.
You can tweak the code to determine your exact volume threshold and how fast you want the volume to change (gradually or all at once). Eventually, Bitar also wants to add the ability to automatically increase the volume again after the commercial and add support for more television brands.
You can find more details on the setup plus links to the code and remote control protocols on his tutorial.
Don't want to build your own? You can buy an automatic volume controller to do it for you for about 20 bucks.