Once he arrives, he knocks and then pulls a key from underneath a small statue of a rabbit (it's not a white rabbit, but rabbits keep showing up in the show). Also, the house number is "233" and 23 is one of the numbers. Once he enters, the shots are very precise to keep any family photos in the dark and out of focus so we can't potentially see who David's mother could be.
Jack goes up to David's room which is full of musical instruments (including a keyboard, guitar and trumpet), music posters, sheet music, CDs and other stuff. Jack stops and looks at the sheet music for Chopin's "Fantasie Impromptu" which is sitting on the desk. There is also pictures of Jack and David together from a photo booth, again we see David is a Dodgers fan because he's wearing a baseball cap.
Jack listens to the answering machine messages in David's room. The first reveals his location. He is at an audition at the Williams Conservatory. The second message is one from Jack while he was in Australia. He only reveals that "something happened" and he wanted to hear David's voice.
So who do you think David's mother is? It seems that this is a mystery that's going to pay off sometime soon, because it has to.
