audio recording using the computer sound card

I am using xp pro os. When I try to record audio with mic, the input for recording is low or near non recordable level. when I select “stero Mix” of the sound card, the input from the mic is improved and I am able to record. When I use “mono mix” for input line, then the mic input is not upto recording level. What could be the reasons for this and remedy for the same. Regards,Krishnamurthi