The sound of the kids at the dinner table–several minutes of nearly total silence while they eat, followed by at least a few minutes of conversation with a minimum of argument. I don’t get this every day, but more often than not.
I also love the sound of babies laughing, but I don’t have any now. My sister has an infant son, and sometimes when she’s holding her baby while I talk to her on the phone I’ll say something funny, just because he laughs when she does.
I also love listening to my two youngest kids greet each other after school. They will sometimes hug each other and then talk about their day.