No audio

I have a home theater with projector and receiver from previous owner.
When we moved in, one of the shelves in the devices cabinet was vacant with 3 Composite cables and 2 audio cables laying on it. They are still connected to the receiver.
DVD player and radio all works fine. I hooked up my set top box to those cables, to check if I could watch my internet TV there. Set top is the only one I have that does have composite out.
I got as far as video working fine via projector but no audio.
Any suggestions? Knowing that outputs on set top box work just fine, I had it running on a different TV. Audio cables are indeed plugged into Audio inputs on receiver.