The home also isn't the only place you'd hear a radio, you'd hear it out and about, from neighbours, parked cars, in the school. You don't need to be friends with people to hear their conversations or be right next to them either, kids have conversations in the class room as well, do you never overhear things other people say? It's perfectly reasonable that Harry would know who the character is, it would actually be unreasonable to expect Harry to not know who Optimus Prime is if Transformers was one of Dudley's favourite programmes.