Great timing! I just bought an external USB sound device to experiment with. Cheap ($25USD) and protects my laptop motherboard.
Did you notice the 1N4001 limiting diodes?
It is "cheap and nasty" but it works and I have found it extremely useful.
I did notice the diodes. They seem to be going in both direction, what is the theory behind that.
Here is a link to WinScope, software that uses the soundcard as a scope too. http://www.zen22142.zen.co.uk/Prac/winscope.htm"
They have info regarding the microphone and line-in voltages:
"The maximum input amplitude should never be higher than 10mV for a Mic input and 2Volt for the line input."
I assume the diode are a way to deal with this, no?