If you can use the microphone in other applications, then you don't have a problem with the sound driver, and it might be just settings issue.
So, you need to check the followings:
First check Windows 8 privacy settings
- Go to PC Settings
- Then, choose Privacy
- Now, click on Microphone
Make sure that Let apps use my microphone
is On, and then make sure that Skype option is ON under Let these apps use my microphone
Now, start Skype and go to Skype Settings. Check the Microphone Settings and try it out.