Serial settings are 115200 8n1. But the protocol is very convoluted and difficult to send manually.
From what I gather it looks like the radio is muted by default. The only site I can find says a 20k resistor from pin 4 in the radio is the mute line. If you want to try that I’d say pin 4 to ground should do the trick. If not then to 5 volts, but I wouldn’t try that just yet. I need to do some measuring on the radio itself. You could do that if you wanted to. Use the stock controller and measure the resistance from pin 4 to both ground and 5v. If you get somewhere near 20k then that’s what needs doing.