First, connect your headset/headphone/earphone with a microphone.
Click Finder
> Click Applications
> Open QuickTime Player
> Click File > Select New Audio Recording.
Click the down arrow 
to select the microphone input.
(If your headset is plugged in properly, you will see Built-in Microphone: External Microphone)
Put on your headset > Click the record button > Speak for 10 seconds > Click stop button > Click play button to play your recording.
Go back to finish troubleshooting the issue.