I’ve just set up Volumio for the first time. I tested it was working by plugging my headphones into the 3.5mm jack on the Pi 3 and it worked great. I then connected it to my hifi Aux input using a 3.5mm jack to RCA cable. Unfortunately I get no sound through the amp. I’ve checked again with the headphones and everything is working fine. I’ve also checked the cable by plugging it into my phone and that is able to play music via the amp with no problem.