Then you're better off using an external speaker with talkback volume control already in it.
Else you could do up a circuit that is not hard, just tedious because of what you'd have to do to install it.
Any particular control on the front panel you wanted to use...?
Here's the concept...
Try to get a 1/2 Watt or 1 Watt variable resistor to try this as an experiment first - to find values you need to make the radio talkback work properly so you don't burn out the control - I would use Resistors of 2-watt minimum to get an idea of how much loudness and heat they will produce.
The Buffer resistor is important as it is the one that will keep the pot form sinking all the current and burning out from the power being applied across it.
That is the purpose of the Shunt resistor - say 150 ohm, 2 Watt to start.
The Buffer Resistor is the one resistor you use now, to handle the volume from the Audio Amp.
The Shunt Resistor usually is other half of the OHMIC value you'll need to quiet the speaker down enough so you don't hear it thru the Pot. So if you use a 1K pot, with that Buffer resistor of say 27 ohms, you'll need a resistor of 120 to 150 ohms 1 Watt or more - to drop the power going to the speaker so the pot when it's fully down, you have quieted the speaker and not put the pot at a point where it dissipates all the power thru it.