How do you turn the compressor on for external signals? Or do you need a special cable I was using a normal quarter inch jack.
I think the opinion around here is that the input signal is too low to get a good mix with the compressor. I’ve asked before but haven’t heard back from anyone with any success. That’s not to say anyone hasn’t; it’s just my experience from trying myself and asking.
The compressor is always on for the external signal. The external input is a stereo input btw, if you plug a 1/4th inch mono jack you’ll only get the left channel…
Also, you need to give a really high gain signal to the external input in order to reach a usable range with the compressor IME. Not impossible at all, but you might need some external gain device (preamp, mixer etc) in order to output enough gain into the input.
I have no sound at all from the inputs not even with a mixer!
Something’s certainly wrong as AFAIK you cannot even mute the external inputs? Perhaps a technical fault… 
Recheck all connections. But I fear that your AR might need servicing if the inputs really are silent…