if you want to change the sound volume you have several options :

• use a receiver channel
• use default value
• set it with the serial USB cable

change volume with a receiver channel

This is probably the easiest option:

connect a receiver channel on the pin "vol" of your board. Then you can cahnge the volume with the potentiometer of your radio; Easy

use the default value

If you do not connect the Vol Pin to a receiver channel, this is detected by the ESP32 and it will set volume to the latest received value.

To be sure to keep the latest value you should remove the power to your board (VCC pin) while the radio is stiff on and conencted to "Vol" pin

set volume with the serial link

You can use your smartphone to connect it to the usb port of your ESP32 (with an OTG cable) or to the Serial port of your PC with a regular USB cable.

Then ope a serial terminal. It can be the arduino IDE on a PC or the excellent Serial USB  application on your android phone.

Then enter the command : Vol=xx or VOL=xx or vol=xx with xx between 0 and 30 to setup the default volume.