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:
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:
Also, check out Naturesongs, which has lots of recordings of different types of creatures in this and other habitats.