You have probably seen those little dongles that let people swipe their credit card on their smart phone or tablet. These little dongles transmit data to the device via the microphone jack. The audio recorded is then examined and the data is either a one or a zero.
In the example described by the article, they are using a PIC and the data is encoded via manchester encoding. A resistor and a pot are used to tweak the output level. This is a great way to send data to a smart phone or tablet without making any crazy custom dongles but you are limited in capabilities.
Schematic and example source code is included in the post.