rear parking sensors - 2012 E63 wagon

[jonboyE63]

I've had my 2012 E63 for nearly a year & I've not found a way of increasing the volume on the rear parking sensors - the fronts are clear & loud but the rears are way to quiet & when the kids & wife are in the car I have to watch the radar in the rear view mirror to ensure I don't hit something...…
Can anyone give me advise - I've looked through the huge owners manual but I can't find any info regarding increasing the beep tone..
Thanks

Jon