Microphones and earphones depend on vibrating diaphragms to convert sound into electrical signals and back to sound again, so you can speak into your earphones to record audio. The audio quality of re-purposed earphones is inferior compared to purpose-built microphones, but they can be improved slightly using audio settings in the computer.
Basics of Earphone Mic Usage
While earphones and microphones operate on the same principle, they are also optimized to do their job well. So your earphones can be used as microphones, but not good ones, and your microphone cannot be used to make a good speaker, either.
Most earphones and earbuds feature a four-band 3.5mm TRRS connector (with four sections separated by three insulators), which provide an additional audio channel such as a microphone input added to the stereo output; TRRS plugs work well in computers with integrated microphone and earphone jacks. For computers with separate microphone and earphone jacks, an external microphone adapter should be used to convert the 3.5mm TRRS plug to a three-band 3.5mm microphone input connector (with three sections separated by two insulators).
Due to design limitations, you will be required to talk loudly into your re-purposed earphones or earbuds for sound to be picked up by your computer system.
Use Headphone Mic on PC
Find the microphone, also known as audio input or line-in, jack on your computer and plug your earphones in to the jack. Type "manage audio devices" in the search box and click "Manage audio devices" in the results to open the Sound control panel.
Click the "Recording" tab on the Sound control panel. Continuously blow on or tap your earphones and watch for green bars to react, indicating that your device is picking up the noise. After you confirm that your improvised microphone is listed and working, select it and click the "Set Default" button. Click the "OK" button, and you are ready to use your earphones or earbuds as a microphone.
Adjust the Mic's Loudness
To improve the input sound loudness, adjust microphone levels by selecting your microphone in the "Sound" control panel and click the "Properties" button. Click "Levels" tab and move "Microphone Boost" slider to adjust levels. Moving to the right will make the microphone louder, while moving to the left makes the microphone quieter. A warning is that adjusting the Microphone Boost levels too high, especially when talking loudly into the earphone, will produce clipped sounds.