Audio equivalent of QR code

As mentioned in my post on coupons, there is a need for an auditary equivalent of a QR code: a pattern of sounds that can be interpreted via a computer with a microphone to provide digital information from an analogue audio source.

For example, during a radio show a presenter could tell the audience that he/she is about to play an audio code for a website. They turn on the microphone on the device they are using. The audio code is played, and the website pops up on the audience member's device.

Potentially, an always-on microphone could automatically pick up the audio codes.