As a loud typer myself, please let them know. I'm usually wearing my headphones and have no idea how loud my keyboard sounds are - and if I'm in a bad mood, it tends to get louder, without me being consciously aware of it. I wouldn't be offended if someone pointed out how much noise I'm generating, and I doubt they would be, either.
This would work especially well if your boss is willing to spring for quiet keyboards for them, and if they're willing to use 'em.
Yeah, back in high school people would constantly point out that I type too loudly. I think the Apple keyboards have helped a bit, still a bit noisy but not annoyingly like I probably would be on other keyboards.
