SB Extigy weirdness

Hi, I'm trying to use an SB Extigy (from around 2001 or 2002) to tune
my car audio. I have windows XP. I have some software that will
generate a signal and measure the response. However, there are two
problems which I'm not sure how to fix:

1. When I plug a microphone into either mic in or line in (I have a
preamp), it is amplified and comes out the output. I want to separate
the microphone input from the output so it doesn't feedback.

2. Even when I have the microphone unplugged, it seems to read the
speaker outputs anyway. Even when I have *both* speakers and mic
unplugged, it still is reading the output! For example, there is a
white noise generator. When I play it I can see the white noise on the
screen. If I unplug the mic, it still appears. If I unpleg the
speakers, it still appears. When the mic is plugged in it influences
the waveform as it should, but it's definitely not ZERO when it's not
plugged in. So in other words, the sound card seems to be feeding
itself internally.

How can I fix it so the input and output are completely separated?

thanks
ms