Through the Door

After talking to the therapist, when his mom is talking to the therapist and Oskar is listening to the door, he's listening with a stethoscope, why can't he hear all of what's being said? Granted I have no past experience with this so I wouldn't know. But it just seems like he should be able to hear the whole conversation. Maybe it was a stylistic choice of the author, to let the reader try to fill in the blanks and interpret it however.