Easy. The Interstate Commerce Clause. Television Ads are Interstate Commerce and as such can be regulated. So this would be one instance were Congress would be justified in using the Clause.
Unlike them sticking it in every 'hate crime bill' or other non Interstate Commerce 'pet project' Bill they can think of.
I asked about regulating the volume of televisions, not the commercials that are broadcast. Two very different things, and it points out very nicely how we have grown to just accept Congressional over-reaching into just about everything they touch.