Built in computer speakers don't work!

Hello,

i have a fujitsu lifebook n series running off of vista. my speakers will work when i first turn the computer on, but once i am logged on to my name, the speakers won't produce sound. when there is supposed to be a sound, however (ex: i get a new message in my inbox), the speakers make a loud thud sound. when i ran speaker tests on my computer, nothing showed that anything was wrong.

please help!