I can if I am in that inbetween phase where you are receiving sensory input form both worlds. But it takes quite alot of effort to break through the fact that your physical body isint resonding to your brain, so it comes out as a breathy whisper. My friend asked me something, he was in the dining room, and I had just dozed off on the couch, It took alot of effort like I said and a couple tries but I managed to get out "Still sleeping"

As for when I am fully in the dream world and lucid, I dont have anyone to check this with me, I am a VERY light sleeper so I suppose I might be able to hear and understand somone, I will have to give it a shot some time.