There was a long thread about this a few years back
How to increase chuff rate on LGB 65000 sound module - G Scale Central
But I don't think there was a simple answer as it got a bit of thread drift about smoke units etc, but worth a read.
Are you able to change CVs on a decoder? That seems to be essential to move forwards.
The manual says there is a decoder in the sound unit. Do you have the wheel magnet sensor installed? If so there is a way to increase the number of chuffs per rev from 2 to 4, and to adjust the wheel diameter which would in turn change the rate of chuffs. That should be in the wheel sensor manual which I can't track down, but it is set in CV64 somehow, not described in the 65000 manuals.
If you don't have wheel sensor then it seems to me (I don't have one of these so I am deducing from the manual) that you need to adjust the speed setting CVs CV2 and CV5 so that the speed of the chuffs matches the speed of the wheels.
Does the loco you have fitted this to have a motor decoder or is it analog? Here's where it gets even trickier. One version of the manual for LGB 65000 gives different CVs to change the speed if you are analog (CV 58 and CV59) or the "standard" ones if you are digital (CV2 and CV5).
So if your loco is analog try changing CV2/5or CV58/59 to change the speed at which chuffs start and rate at max speed
If you loco is digital, isolate the motor decoder then change CV2/5 as above.
Bit rambling but hope this helps. I see someone else has posted while I write this, I hope it says the same sort of thing or has a better answer!