Meet the Howlers was just released. To listen to my howler monkey sound recordings, check my book page, here. National Geographic also has photos and a bit of information:
http://animals.nationalgeographic.com/animals/mammals/howler-monkey.html
Some better sounds are on an NPR story about Greg Budney, the noted sound recordist. It takes a while for the NPR player to load, so be patient:
http://www.npr.org/templates/story/story.php?storyId=92926828
Also, check out Naturesongs, which has lots of recordings of different types of creatures in this and other habitats.
http://www.naturesongs.com/costa.html
Happy Howling!