From what I can tell, the cable has a built in attenuator so the issue most likely is within the camera itself in its automatic level control.
In your posted recording, one can hear the gain surging upwards when there is no transmitted audio. That is when the hiss comes in which is amplifier noise - most likely coming from the audio panel - but being amplified many times which is why this is not heard on the AC speaker or headsets.
Is there not a line in available on the camera?