How to Get Audio Line-Out to Work?

Hey guys, I plugged in my speakers I used to have plugged in to my Mini Mac to the audio line out on the back of my new Mac Pro, and it doesn't produce any sound out the speakers. Yes, I know how to toggle preferences for audio line-out. Makes no difference. When I select "line-out" in preferences, my speakers will play through the front headphone jack, but not the audio line-out on the back of the machine. Any tips? Why does audio line-out not work like it did on my Mini Mac?

 

I see. Thank you, I bet you are correct. I'm waiting on some Swan M10s, so hopefully those will work with the lin-out jack on the back of the Mac Pro.