It should work when plugged in to the front slot, as long as the speakers are set up properly and turned ON. I can see that the power switch of your speakers is on one of the speakers.
Also, regarding the back ports or slots, they are intended for individual speakers.
Here is the layout of the ports and what they are for:
So if your speaker system only has 1 output port, it is not intended to be using the back ports, it should work in the front port.
I hope that helped.
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