Use Jack Audio Connection Kit to monitor your microphones

When building a product with built-in microphones, one important but overlooked diagnostic is to just listen to what the microphone is outputting, live, in real-time.  It seems silly that I even need to mention this, but everybody falls into the trap of just listening to recordings or worse, not listening to recordings!  There are so many artifacts and effects that you hear if you're playing around with the device live, rather than in a recording.

If you are on Linux, and you have a sound card that connects to your microphones and to a headphone, it's really simple using the Jack Audio Connection Kit.

1. Install qjackctl

sudo apt install qjackctl

2. Then start 'er up:

# qjackctl

That will bring up the qjackctl window:

3. Click on the 'Setup' button:

Make sure all the settings are right, especially your interface.

When you're happy, click OK.

4. Click 'Start'

Once you've started, your window should look something like this:

Everything started okay, only one last step to listening to your audio:

5. Connect inputs to your headphones

Click the 'Connect' Button.  It will bring up a window that looks like this:

My sound card has 34 inputs, and 16 outputs.  In this case, I'm listening to microphone 11 in both ears.  (My headphones are on output 1 and 2).

The only thing to remember is that if you count starting at 0, the channels in this dialog are all 1 higher, because like Matlab audio channels in Linux start at 1.  Ugh.  Oh well.

Caveat

There is a little caveat -- there are 2 main releases of Jack, Jack 1 and Jack 2.  There's some history there that I don't know much about and don't want to.  It's possible that your system release may work with one or the other only -- it's a headache, so there is some possibility you'll need to recompile jack from sources.  But it's pretty fast and easy.   Just hope your system works fine as-is.