Check the control pod pinout corresponding to that channel (e.g., Pin 1 for Center, Pin 6 for Rear Left) to verify that a signal is successfully passing from your control source into the HD15 input port.
: Carries the analog signal to the internal amp. logitech z5500 wiring diagram exclusive
Three 3.5mm stereo mini-jack cables (or six RCA cables, depending on your source). Check the control pod pinout corresponding to that