I'd check out this thread. Might be something that a dealership did to help this person adjust the beep of the lift gate. Apparently it can be turned off - I'm sure the reverse is true but might just be something the dealership can do:
www.ascentforums.com

How to adjust power liftgate beep?
Hi. I can't figure out how to lower the volume on the beeps the power lift gate makes when opening or closing? If volume adjustment is not possible, then I'd like to turn the beep off. Anyone out there know how these things can be done?