Are "low talkers" rude?

My boys, 23 & 19, live in a small 2-bedroom apartment, which has a small kitchen/living area. The question is whether it is impolite / rude for them to drop their voices while I'm within earshot (5-10 ft) to a level that I can't hear. I told my older son that this bothered me and was considered rude. He responded that he sees nothing wrong with it and that I'm making it up.
Thoughts? He might believe you.