I guess my very favorite background sound is of my older child (Leia Rose) trying to teach my younger child (Annie) anything and everything. Most recently, Leia was trying to explain to Annie why a character in a book would be bewildered enough to mumble a non-existent word. (In this case it was Mr. Dursley of the Harry Potter series mumbling "mimbywimble" as Hagrid explains how Harry is a wizard.) I love to hear Leia's methods, ... and even her frustration ... as she shows such great love for Annie.
On a more humorous note, Annie has one of my favorite sounds, too: screams of animal names. Ha! Let me explain. Last Christmas, I made Annie a book that would compliment an unusual interest of hers: animal skeletons. Each page has an animal skeleton with a flap that shows the living animal. Annie LOVES to go through this book, scream out the name of the animal, then lift the flap to see if she is correct. I love this sound so much, that I actually took a video of Annie doing this crazy thing. Ah, the memories!