Yes, the speaker leads connect to the microphone. I'd suspect you'll need a resistor network to cut down the voltage levels to microphone sized inputs. A series resistor of around 1K into a selected value resistor to the amplifier common (not frame ground). The resistor is selected to give you microphone level voltages, since I've never actually used this rig, I can't say for certain what the exact value of the resistor to ground might be.
Also, since the speaker leads float with respect to frame ground, you'll also need a fully isolated 5V supply to feed the transmitter module, there are a number of DC-DC fully isolated converters available for a few bucks.
Here's one at Aliexpress.com that's a 12V to 5V DC converter and puts out 200ma of 5V, more than enough for the transmitter, search for item 3256804874735956
Receiving side.
The good news is, you don't need isolation for the receivers since they're really stand-alone an just receive the over-the-air audio. Just a three or four component 5V supply will do the trick there.
Item 2255800209759634 is a suitable amplifier module, it also runs on 5VDC and will drive your 8 ohm speaker directly.
