I can't hear sound without install sound driver but i can see without install graphic driver.may i know why? somebody help me!

Most OSs will have built in, basic video drivers. Good enough to display a screen. You would load the specific drivers from manufacturers to realize the full potenntial of the card.

Sound is not an essential part of using a system. Older cards might be able to take advantage of some generic sound drivers that ship with some OSes, but usually you want the manufacturer driver anyway.