You can't adjust the volume level.You can do a system reset by disconnecting the battery for 10 mins or so.
You will then need to re-programme the power windows and the car may not run perfectly until 2/3 miles.
Otherwise it's a visit to the dealer i'm afraid.
Slightly different issue but my son's car, a Kia Ceed, has an on/ off switch for the reverse warning beep. Maybe Ford could consider something similar, because to be frank if I had a car full of people and they were all talking whilst I was reversing, I couldn't hear the damned thing anyhow and would probably be better turning it off.
That's one option, I suppose, if I don't mind falling out with them. A further option would be disabling it altogether.
I'm not really a fan of these parking aid thingies because I reckon they make you lazy. But then again, I don't live or work in a city where parking opportunities tend to be limited and when they are found, they tend to be tight.