My brother is having issues wiring his pc to the audio system. He is splitting the audio from 3 3.5mm audio outputs on the rear IO of the motherboard into dual rca adapters and then wiring them into the speaker system. The pc cant detect these as being audio outputs and so he cannot set them as the audio output device. I don't know much about speaker equipment so please no heavy answers however I am competent with pc's. He is using an old asus motherboard, unaware of the model, and is running windows 7.
You need to find out the model of the motherboard and the model of on-board sound chip.
Most Realtek HD chips allow you to change what each output does.
For example you can change the pink output from MIC to Rear Speakers.
So if it's Realtek, make sure you have latest driver and try double clicking on output pictures within realtek control panel to change the settings.