1) Most commonly this problem is caused by 3rd-party sound enhancing software, for example Nahimic or Sonic Suite.

In order to resolve the problem please check your Windows system tray at the bottom right side of the screen - if you see any audio-enhancement program icon there, right-click on it and close the program. 

Please attentively check the list of your installed programs (Programs and Features) - you might just be unaware of the app being installed on your OS.

After that restart Screen Capture and try enabling audio devices (System Audio and Microphone) again.

2) Try updating sound card drivers of your current sound device.

3) Sometimes the problem is caused by difference between the sound quality settings in the screen recorder and system.

Right-click the speaker icon on your system tray and select Playback devices. 

Try temporarily disabling your sound device. 

Right-click your current playback device and select Configure.

Select Stereo option.  

Click the Next button, then Next again and finish configuring the device.  

Right-click your playback device in the list and select Properties.  Go to Advanced tab.  

Set the following settings: 24 bit,  48000 Hz for your speakers.  

Tick all the options under 'Exclusive mode'. 

Click Apply and try recording the audio once again.

4) Please note that if you have Windows 10, your permission might be required in order to start recording from microphone.
Please find more details here.