Mi DDJ FLX 4 cannot be used as an audio interface

Been trying to connect my DDJ FLX 4 using the RCA connections. The thing is, in Rekordbox everything is working, and the signal is going out, but I don't know how to make it sound. There is no music at all.

Al ready changed the outputs, and change the audio device. 

Hope someone can help me. Don't know if my problem is with the FLX 4 or something in Rekordbox.