First of all I'm not completely sure this is impedance mismatch, but from what I found on the Internet I believe it is. It seems to be a common problem. The question is not as much about solving the problem as about why it is happening (if I'm right about the cause of the problem, of course).
I had this quiet microphone problem with several built in cards and microphones and now with a Creative Audigy SE. There's a microphone boost option which introduces a lot of noise with volume increase, but even this doesn't seem to give loud enough sound in some cases.
The mic on my current headphones is very quiet with Audigy SE without the boost but is very loud and low noise with an external Sound Blaster Connect.
So the question is have I just been unlucky with my sound cards and microphones or is it a common problem? And if it is a common problem why is it so difficult for the vendors to standardize on the sound card / microphone impedance?
Edit: the OS is Windows (XP/7), but I don't believe it is OS-specific.