"Audio source - Select the audio source for recording video. The effect of this depends on your device - if it supports an external microphone, you may be able to use this by selecting "External mic". The other options may provide different settings affecting the resultant audio (e.g., automatic gain control), though this behaviour is device specific."
Looks like its for android devices that have things like a microphone jack on them.
Due to the vagueness of the description, my guess is the guy built in a pull down menu for the android sound mixer and is simply letting the android sound mixer allow "what ever". In most cell phone generic cases this would yield nothing special, but if it was a non-cell phone android device that included external audio inputs then they would be accessible here.