You might want to use another Recording Program (Game Cam)
This may help too:
FRAPS show fps, record video game movies, screen capture software
"How can I adjust the volume of the sound that is recorded?
You must configure the volume through the "Sounds and Audio Devices" item in the Control Panel. Use the following steps:
Click on the Control Panel and select Sounds And Audio.
Click on the Audio tab.
Find the Sound Recording device and click Volume.
Adjust the volume for the Input that Fraps is recording from. This is usually named "What U Hear", "Record Master", or "Stereo Mixer".
If you can't see the correct Input on screen you may need to enable it by clicking on Properties in the menu and ensuring all Inputs have a check beside their name."