A few problems I've found with microphones and PC sound cards...

First off, the microphone may just not be giving a big enough signal out. I bought a cheap mike for talking into for the PC, and that needs a separate preamp to work with anything. If the mike is an active type (ie. it has a battery in it) then this shouldn't be a problem, although then the problem may be a too-large input.

Second off, sound-cards mounted in a PC just aren't a good move. Digital electronics is very electrically noisy, especially at high frequencies (the metal box is to keep the noise in, not to keep external noise out!) And into this noisy environment we're sticking a soundcard and expecting it to work .. the surprise is that we get anything decent at all!

Thirdly, you get what you pay for. As I've found out to my cost, all-in-one motherboards with the sound card included don't work well - they're even closer to the digital bits and suffer very badly from noise, and they're probably not too good in the first place.