Need LED Circuit Design Expertise

I have quite a few radio scanners in a row in a newsroom. I'd like an LED to light when there is a broadcast and stay lit until the audio is gone on a particular scanner when there is audio through its speaker. Speaker output is anywhere from a few to few hundred millivolts AC (of course) and I've tried a voltage doubler but the LED only flashes on the peaks. I need it to light up when the audio is active, stay on during reception of the call and go off when the receiver goes quiet. The LED action will help us tell which receiver the call is coming through. I have 12VDC and speaker-level audio available at each receiver. I actually have a budget to pay for circuit design if interested. Maybe this is a PM kinda deal? Thank you for your time.

If you can perform minor surgery on the radios, I would suggest acquiring the service manual, or at least the circuit diagram, and having a look at the squelch circuit.
There will probably be some point from which you can switch an additional transistor to control an l.e.d. on each receiver.

Surgery is not a problem although finding a S/M might be. I had hoped to keep this external as there are different scanner models. Thank you.

I would agree with Syncopator. If you try to use the audio to decide if a scanner is active you run the risk of the LED flickering on and off in time with the sound and unfortunetely if the radio link is FM, it staying on when the channel isn't in use because it would see the background hiss as valid audio. The best method is to use the squelch or AGC signal in the scanner so it alerts you when the transmitter is turned on, regardless of whether it is carrying audio or not. Basically the LED is a 'signal present' indicator.

Brian.