There really isn't alot of people who can LEARN to growl
I'm inclined to disagree, at least from my own personal experience. I never knew how to growl until I started listening to wolves growling and other people showing/telling you how to growl. Only recently have I gotten more used to it, but it still needs work. I practice on my stepdad's cat occasionally as a kind of mark of progress, and it serves to scare him now
Plus I've tried growling at other dogs that have started growling at me, and after a bit of a canine standoff, they yield.
As for howling/barking/etc, I'm not sure if I can help. I'm still trying to correct my howls...And as a naturally shy (read: practically silent) person, barking doesn't work so well because I just have trouble summoning up the vocal strength for it
A similar predicament occurs when I try metal growling/screaming.