Pioneer XDJ-RX2 / External Input

So, about a year ago I bought a XDJ-RX2. The first months that I got it, I didn't touch it at all because I didn't know if I would be able to get started. Now, a few months later, I finally started to begin with this product. Just yesterday when I opened rekordbox, I got to see that one of my channels (number 1), was changed to external input. I didn't think it was serious but when I tried to play song via that channel. No sound was produced. I don't know what to do so I came here.