I am trying to run the headphone out of my Intel iMac to a mixer (Behringer UB802) via a 1/8" to 1/4" cord. Then to an external set of studio monitors (Alesis M1 Active). The problem is there is no sound when I do this. When I plug headphones into the headphone there is, of course, sound coming out of the headphones. Does anyone know what my problem might be? I did notice that the plug-in for the headphones has 3 different "sections", and the plug-in for the cord I am using has two different "sections". Thanks for any help.