Just kind of a shot in the dark here
What does it look like when you hilight stereo mix and hit properties? If you see an enhancment tab, check and see if there is any kind of reverb checked. Is this an onboard soundcard or a soundblaster style? yours will look a little different but this is what I what like you to look for
Look at the properties for both the recording and playback devices. this is my playback device, I am not at home so I can't look at recording (I don't use one here at work) Also check in the Realtek audio manager to make sure no enhancements have been selected.