Macbook Air microphone
In Skype, people cannot hear me. Skype voice test indicates mic does not work
You should have two easily accessible mics - one built-in and if you plug in any earbuds with an in-line microphone (like EarPods) you'd have a second.
The quickest way to verify you have signal and adjust their levels is in System Preferences > Sound > Input
Double check the audio input. If the system is getting any audio signal it will display the levels there. Once you know the system is getting audio via System Preferences, you'll want to verify Skype is setup to use the default (input specified in System Preferences) or the specific mic you'd like to use.
Here is my setup: