You want to record only the audio, not any video at all? Try using Audacity[1] in stead, that is audio specific, and has the ability to record. To export as MP3, install Lame.[2] Finally, if you haven't already, you need to activate stereo mix[3].
If you absolutely want to use Camstudio, record a tiny part of the screen, and extract the audio from the video with Virtualdub[4] or similar: Open the video in Vdub and select File --> Save WAV. (If my memory serves me right.)