You know how to install windows then there is no need to take it to some PC technician, you are asking your problem from a technician right now (wase bhe apne peero ko kon manta hai ) and its just a minor problem

go to my computer (right click) properties> device manager > sound. See if there is exclamation on any driver, if not then

I will recommend to download windows Xp (S2 or S3) form any torrent site you like. Then burn it via any disk burner application, I will say Nero 9..(its easy to burn disk, if not then google it)

After the disk is ready, the rest of the process is same, insert the disk and install. then install sound driver..if that still dont work..then check your computer hardware and especially the speaker wires and sound jack..