I don't think this is an OS problem as much as a physical hardware "problem". Your comparing phones, they do not use the same hardware/technology as old phones, I've noticed the same thing, but they use new and different hardware for new phones so you can't expect it to be the same, unfortunately.
I had an old curve (8310) and also noticed that the vibrate/volume is much louder.
But that's the swap you make for new technology, no?
I guess try and update the OS, but I highly doubt that will fix your "problem".