Hi everybody, like i said, I can't get the audio out of my speakers using Serato DJ Intro, i have a DDJ-SB2 controller, and a 5.1 speakers, the speakers have 3 RCA plug (1 for the rear speakers, 1 for the front speakers and one for the center and sub), and i connect to my pc just via the USB cable that goes in the rear of the controller.

My controller works with serato, but I can't get any sound of it,

I suppose I must connect the speakers with the 2 RCA plugs in the rear of the controller, but my 5.1 spakers have 3 of it, and not 2.
How can i solve it? do I need some adapter or external sound card?
Hope you guys can help me