Nice article. I totally agree with the usable aspect of sound here. This reminds me of a probably slightly less tasteful topic: flushing sound buttons on Japanese toilets. Google this and you will get plenty of descriptions.

Basically, people are ashamed of certain sounds while seated on the toilet. Having lived in Japan, I must say that some toilets there really don’t offer a lot of auditory privacy. The result is that people mask the sounds by flushing several times. In a kind of bizarre design loop, the toilet makers consequently added a flushing sound button to reduce the consumption of water.

Now, that’s what I call an interesting use of sound.