Never thought about this, but the listening part is actually quite nice addition. For example my fridge and freezer both use "beeps" to signal something is wrong. My stove, oven and dish washer are also using beeps (but a little bit different) to tell the food is ready or dishes are done. My Neato uses beeps to signal it is in trouble. My washing machine does not beep, but it makes a specific sound when it is ready and the lock on the door is released. If the bed room windows are left open, the sounds of traffic are more loud than usual. Also all my doors make a specific sound when they are opened.

Now when I think about the "beeps" would be actually quite neat and simple way to integrate various home appliances to this kind of monitoring system. Not as cool or comprehensive as bluetooth or zigbee, but simple to add. Intelligent listening could probably reveal all kinds of interesting stuff without the need to adding new sensors here and there.