Chirp.io uses sound to transmit & receive links between nearby Android/iOS devices. IMO, interesting tech[1], thats more useful & less secure than bluetooth, for such use cases.
Chirp is doing some really interesting work in this space, trying to improve the user experience of fast-forming short range communication.
My understanding is that Chirp uses audio to basically detect WHO you are standing near, but the actual data transfer happens over more typical links.
"An inherent limitation of the audio protocol is its highly limited transmission rate.
To send larger amounts of data, we have built a RESTful network infrastructure which allows arbitrary pieces of data to be associated with Chirp shortcodes. A sending device can thus upload a photo to the cloud, and obtain a shortcode representing it to be send over the air. A receiving device hears the shortcode over its microphone, and resolves it with a GET request."
[1] http://chirp.io/tech/