Camstudio makes AVI video files, not audio files. If you record just the audio in CamStudio, you will get an AVI file with video of the screen at the time the recording was made plus the audio.
To record audio only, use an audio program like Sound Forge. Start Sound Forge in record mode and anything that goes through the sound system will be recorded in the .WAV audio file it makes. When the recording is finished use the edit features to crop the ends off and adjust the volume if needed.
If you want to copy the sound off a video file, just play the video while Sound Forge is in record mode. The output file will be a .WAV file. It then can be converted to MP-3 if you want.