one rule of thumb to remember when it comes to safety devices on equipment.
Wether it was OEM installed or aftermarket, doesnt matter, if its on the machine it has to work.
In todays work environments some type of movement alarm (especially in reverse direction) is a requirement and depending on the work envrionment the working mode of the device varies.
Most if not all machines built today have a programmable alarm built in so it is required to be operational at all times. You cannot remove it without express permission from the manufacturer (which i doubt you will get due to OSHA requirements).
If you want to modify the device you should not do so without prior permission and approval of the modification from the manufacturer, and be prepared to give them every technical aspect of the said modification for them to be able to make such a decision, they will ask , trust me ;o) (engineers are usually very resistant to any modifications of their designs anyway).

although there is not an official 'forklift police' running around checking for these things it has been a well known fact that an accident caused by these things being done that triggers an OSHA investigation usually results in someone getting a huge fine.

Keep this in mind before you think about modifying, disabling or removing a safety item that is installed on a machine.