If I may, you shall keep in mind the "KISS" concept (Keep it Simple and Stupid, don't complicate things)
Why would you need a soundcard? Use an analog amplifier to get your wanted signals.
The amplitude range will fluctuate? Use automatic gain control. Chips like compressors (I have the SSM2165 in mind) will help you to handle multiple input levels within time.
The issue to the accuracy relies in the analog electronic world.
When you'll need processing horsepower for let's say high precision time/frequency transformation, you'll think about adding a powerfull soudcard.
But first, dig into the analog world.
Regards,