Introducing myself and a question about ddj sr

Hi guys, my name is Hadi Mahfouz and I live in France. I've been mixing mostly house music and little bit of hip hop and rap since 2 years. I still never mixed in a bar but planning on it. Right now I am planning on taking some courses and look for some tutorials to take my skills to another level.

Now for my question. I've recently baught the ddj sr, a really nice controller. When I plug it to my computer, plug the sound system jack to the controller, and I open Serato, and I play a music on the program, everything is fine, I can hear sound going out. However, if I want to play a video on youtube for exemple some tutorials, with my controller still plugged in and Serato still open, I can't hear anything, I have to plug out my controller and put my sound system jack in my computer.

Do you know how can I fix this?

Thank you for your help.