The microphone you are talking about is the headset microphone? When it worked fine yesterday, was that before the new sound card was installed? It's only with the new card that it is giving problems? The headset mutes the speakers?
It sounds like the microphone is not working at all, not a feedback issue, true? Do you have an app that shows sound levels so you can see if the microphone is registering sound, however little?
Have you checked the controls for the sound card itself (other than audio properties)? From the card specs there are a number of input features that may be affecting the mike audio.