If your audio is completely silent , it is probably one of two cases:
i) The audio device is not capturing audio
ii) The speakers/headphones are muted or not working.
The device is not capturing audio
- The PC/Mac has more than one audio device (webcam, usb, virtual capture etc)
Choose the correct device from the settings area before recording:
- The audio recording device is muted
Sometimes mics have a button, sometimes another app can mute them,
Try recording from the PC/Macs native audio recorder. If it works ok, its probably not muted.
If it doesn't work try adjusting the settings. In the worst case scenario the mic is no good, and you should try another.
- The audio driver has crashed
Sometimes, especially if the PC/mac has different apps using the mic (zoom, skype , camtasia etc) the audio driver can crash or get locked by another app.
Rebooting the PC/Mac is the best solution
- The mic is plugged into the wrong port on the PC.Mac
This could be true when using mics that plugin to the sound board on the computer (ie not USB mics)
Change the port and try again
The speakers/headphones are muted or not working
- Visit Youtube.com and try to play a music video. If you hear sound, all is ok. If not then check your speakers/headphones settings and volume.