Hmmm, I don't know of any way to adjust that. I think it is a small internal piezo speaker/buzzer that is likely controlled with an internal potentiometer. Locating the right one inside the RC may help, but I wouldn't try it. It could also be a hidden combination of buttons and stick or wheel movement that adjust it. I also notice that some beeps are louder than others, or my ears just can't hear all the pitches at the same volume. Without the AC on, you may want to try some of the tones for yourself, i.e., press but don't hold the RTH, Sounds loud to me, press and hold the RTH and only the first beep is loud, and the following beeps are subdued.
Sorry, not much help.