I imagine a use case where you have headless devices (servers, raspberry pis, etc.) that are not connected to the internet. These devices can be measuring something through a sensor and periodically reporting the measured value using sound through the speaker. When you go near the device of interest, you take your phone and "listen" to the emitted value to see what was the last measurement.

The advantage of this is that it is a very cheap way of adding such communication channel to your device.

at one point the iPod bootloader was extracted by playing it aloud: http://www.ipodlinux.org/stories/piezo/