i tried plugging in the singing mic into the computer into the computer.. the sound came out very softly.. <---
your singing mic is a "XLR" balanced mic? and you're using some kinda adapter?
but this situation will not happen if i were to plug in computer mic.. <-- your computer mic is unbalanced most prob.
sounds like impedance problems to me.
yes you need a preamp if you're using an XLR mic.
pls read the DIY recording faq =)