What's your operating system? If you're using XP, there's a little drop-down menu next to the recording levels where you can select the source you want to record from (computer's speakers, or external mic), otherwise, like if you're using 7, then you have to go to control panel, sound, and set the speakers are your default recording source (and you'd actually have to do that everytime you end up hooking up and unhooking an external mic).
Also, if you're using 7, for some reason, recording in 7 seems to be rather sensitive to certain things, like for instance, anytime I record anything in 7, the bass is automatically boosted.