[cryptoz]

There is work being done on crowdsourcing ambient sound: http://www.technologyreview.com/view/520606/noise-pollution-...

It's a neat problem. You can turn on smartphone microphones and listen for noise levels to measure for noise pollution and law violations, etc. However, you have to develop algorithms that filter our human conversations (for this application linked above), and the privacy implications are astronomical.

Edit: http://www.citylab.com/tech/2013/05/crowdsourcing-citys-quie...

[edsykes]

we've also developed an early version of this. Still very early stages and we aren't making use of all the data yet:

http://noise.1self.co

disclosure: I'm a founder