The advertisers and stations have to make sure we can still hear the commercials when we are in the kitchen, bathroom, or laundry room during them. :-)
Here's an idea for an inventor: Most, if not all, TVs have a volume control on the remote. Those usually work by the remote sending out infra-red signals to the TV itself. Those signals are coded. What is needed is an aftermarket device that also sends out infrared signals.
That device would monitor the volume, and send out up-down adjusting signals as necessary to keep the average volume to what you have set.. As with universal remotes, you would "train" this new device with your TV remote so that it would send out the proper signals.