I don't believe that there's any way to convert what's being read from the soundcard into a voltage directly. The soundcard doesn't really care what the absolute voltage of a signal is, just the relative changes in voltage. A better way to do what you're trying to do would be to watch the output of the sound card and see what level it gets to when you want your program to procede. Then you can just watch for this level. Also, in all of the microphones that I've dealt with I've never heard of one that could get up to 5V amplitude. Usually microphones generate a very small voltage. 5V might be a little high of a threshold to look for.
Hope this helps,