I think the first time I realized that I needed to pay a little more attention to Luke's mumbling was during Dora.

They both watch Dora and then Diego before bed every night. Luke loves Dora. One night he's sitting there watching, and Dora asks: Who do we ask for help when we don't know which way to go? And Luke promptly replies MAP!!!! Plain as day. I perk up and go....did he just say "map"? So I go into the living room and listen. A couple minutes later Dora says to them: Say Backpack! And sure as shit he says: PAP PAK! I was totally astonished! About a week later, again during Dora while looking for swiper, she asks them: Do you see Swiper? Where? and Luke em phatically points at the screen and says: "RUT DAIR!!!!!!" (right there) Two words!! IN A ROW!!LOL And still every time something like that happens Mike and I still just look at each other in amazement.