I was facing the same issue as Nicolas. There is little, almost no documentation for ASTRA being used as a microphone. I guess maybe there are better choices in robotics etc.
So lets jump straight to the question.
First of all, here is an image of camera's individual components, so as to get a better perspective of microphone positions :
a) When plugging astra , in pulseaudio GUI (Ubuntu sound settings) ASTRA microphone should be visible.
b) Through alsamixer you can see ASTRA as a standalone "sound card" by pressing F6. You can play around and make some configurations on your own.
c) You can use ASTRA as audio device on audio software (e.g through Python's PyAudio module).
Just make sure your system detects the device ( in cmd "lsusb" and look for the device that belongs to ASTRA).
I hope these comments, to be of help to anyone attempting to use Astra camera as audio input device !