Yes that can be quite tricky. For sound deadening: the method I use: first try a couple of different ways on a small test patch, then do the whole thing. Methods to try: chisel, applying a bit of heat, cold spray and then hitting it with a hammer (surprisingly effective!), large diameter rotating steel brush (not very good for detail work due to the large diameter but it eats through that stuff in a hurry), pushing 'up' from below to deform the floorpan a bit to see if the stuff delaminates (sometimes you get lucky). Pick the one that works best for the initial removal, then do the remainder with the next slower method until it's done. Tedious.

Factory undercoat: rotating steel brushes are the only effective way afaik and it takes forever unless you have access to a large size sand blaster.