If it's adjustable, you should be able to just spin the volume wheel while it's beeping at you. Anything that has a volume level in the settings should work that way, though I haven't tested the parking sensors specifically.
I think I figured it out: In the PCM there's a "Parking" icon/tile and in that there is an option for "Obstacle warning volume" with three settings, loud/regular/quiet... I set it to "quiet" & set it to my profile (# 1) on the door button. We'll see if it "remembers" my setting on my next drive...