Listen at Xeno-Canto

Xeno-canto is a community database of shared bird sounds from around the world.  It's an ever-growing collection of bird sounds.  At xeno-canto, you can listen to dozens of Campephilus woodpecker vocalizations, drumming and knocks from 9 out of the 12 species.  Missing are any sounds of the Ivory-billed Woodpecker, the Cuban Ivory-billed Woodpecker and the Imperial Woodpecker.  That still leaves a multitude of recordings of the rest of the Campephilus family for your enjoyment and study.

http://www.xeno-canto.org/browse.php?query=campephilus

Xeno-canto lets visitors to its site participate by discussing and identifying unknown sounds.  And, if you record bird sounds, you should know that xeno-canto also lets people upload and share their own recordings on the site.

Also, don't miss the highly informative Species Overview  pages on Xeno-Canto.  Available from the left frame menu, these pages offer a summary of number of species sound recordings, photos, range maps and sonogram images.   See this page, for example:

Species Overview page for the Red-necked Woodpecker