My computer (MSI Cubi - http://us.msi.com/product/bb/Cubi-Mini-PC-Kit.html#hero-overview) has a headphone/mic combo port. A single port like you'd find on your mobile phone. However, after plugging in multiple headphone with a mic such as Apple earbuds or Marshall headphone/mic the headphones work fine, but the mic does not work at all. Going to the System Settings > Sound > Input > and selecting Analog Input - Built-in audio it is enabled, but the sound level bar doesn't register any sound when talking into it. USB mics works fine.
Can you check if the microphone is enabled by looking at the alsamixer output?
Open a terminal and enter
alsamixer and press enter.
You may need to select the correct sound chip first, it's probably "HDA something PCH" or similar.
If all goes well you should see various bars, it could be that the audio level of the mic is 0 or that it is muted in some other way.
Things like that don't always show up in the sound settings.
With the Cubi (and presumably other computers) there’s a Headphone icon followed by a slash / followed by a microphone icon. Turns out this slash means “either or.” In other words, you can use the headphones or the microphone, but not both. Who knows why they do this. However, the good news is that a “fix” will cost you less than $15. You’ll need two things: