First model year 2011 Nissan LEAF had a back-up beep similar to that of a construction vehicle, and fairly loud. It also had a dash-mounted factory off switch for the noise. Then came the 2012 with the same noise but no off switch. Then came the 2013 and subsequent models also with no off switch but the ability to be turned off by connecting to the CAN bus. I am totally out of luck having a 2012. And the noise is loud, easily heard inside the vehicle, annoying for neighbors backing out of my garage, and universally disregarded by pedestrians in parking lots. Another mommy-state solution for a non-documented problem.
You could put a switch in one of the wires going to the speaker. Or you could put an audio taper rheostat (variable resistor used to control volume) in one of the lines and adjust the volume. You’ll need to make sure it doesn’t get wet so it’ll need a little box. Anyway it is fairly easy to decrease the volume or to disable the sound.