if you are using a VGA cable to the screen, you would need to take a line-out from the sound-card on the PC to the auxiliary input on the screen

or you could get a HDMI graphics card on the PC, but you would need a motherboard or sound card which has SP-DIF to run a cable from that into the graphics card, then the HDMI cable into the screen would also provide sound.

otherwise, use some surround sound speakers plugged into the PC, they are dirt cheap and sound 10 times better than the speakers on your screen