There is no sound when the volume control knob on the SL-40 is pushed in but I get sound with the knob pulled out. SteinAir sent a replacement radio but the system has the same problem. Stein suggested that the problem might be with the intercom. Van's sent a replacement intercom but the same problem exists. I tried tweaking the COM Pot to no avail. Ken at Van's suggested that it may the headset I am using. I have not tried a different headset yet. Any other suggestions will be appreciated.