It is likely your PC audio settings are not set up right for recording.
Depending on your operating system, the steps will vary. But with Windows 7 you will need to right-click on your speaker tray icon then choose recording devices.
Then you will need to go to the recording device options and look for something along the lines of stereo mix as your sound card’s audio to set as the default recording device.
It’s possible you may even have to enable the device, in Windows 7 again you just right-click on the recording devices tab and check the boxes to show disabled devices.
Then enable the stereo mix or similar option & set it as the default. You can then set ScreenCorder to record audio from that same device.
You will also need to set the recording sound volume up high so that it will record loud enough. If it is set low, then when your regular audio is turned down too, you won’t be able to hear what you recorded.