Do bulls moo, or just cows?

Bulls "moo" as cows do, but you'll get tons more aggressive grunting, 'cause they literally have a pair. Can and do they moo? Sure, just not as likely to happen with cows and steer.
Cows and steer (castrated bulls) have a less aggressive nature (time to talk testosterone with the kids?), so you'll hear more so-called mooing, but what a cow/steer can do in terms of mooing, so can a bull. Just expect the more aggressive bull to grunt and huff and puff more (there might be a moo in there somewhere).

http://boards.straightdope.com/sdmb/showthread.php?t=592449

Yes, they make a similar bellowing sound, often in a deeper pitch.

According to this, bulls bellow:

http://www.myenglishpages.com/site_php_files/vocabulary-lesson-sounds-animals.php

(see the listing for cattle)