Normally the sound card (microphone) is on port 0. (At least on my laptop.)
As far as potential problems, I assume that when you say "record a voltage signal" that you are recording something other than a normal microphone - i.e. connecting up a real voltage. If it has too high a level (or DC offset) you could fry your PC. If that's not a concern, the answers you get are in DAC values, not volts. You need to do your own calibration - which will need to change if anyone/anything adjusts the microphone volume settings. If that's not a concern, note that your PC's sound card may have low and high frequency roll-off, and might not have very good frequency response in general, and might not be linear. If you would like to take a quick look at what your signals look like, www.CompleteTest.com has a PC Scope on its Products page. The PC Scope uses the microphone port as an input and displays the signal on a scope-like display. It is written in LabVIEW 7.0. You can use it free for 10 days. Les Hammer Complete Test
