My dictionaries of first resort, the OED, Merriam-Webster’s Collegiate, and American Heritage, all give a four-syllable pronunciation for the name of this New England Indian tribe; M-W renders it “wäm-p&-’nO-(“)ag, AH is the same (rendered in their own system), and the OED differs only in having a schwa in the final syllable. But the original pronunciation was clearly three syllables; the first citation in the OED (Roger Williams, 1676) calls them “Wampanoogs,” and the ending must be the same as in the original Narraganset name for the Pequots, Pequttôog, and the word for Europeans, Wautaconâug ‘coatmen,’ which I presume are two spellings of the same vowel or diphthong. It was still three syllables in the early nineteenth century; John Greenleaf Whittier’s 1830 poem “Metacom” rhymes “Beneath the closing veil of night,/ And leafy bough and curling fog,/ …Rested the fiery Wampanoag” and “The scorched earth—the blackened log—/ …Be the sole relics which remain/ Of the once mighty Wampanoag!” In 1847, John Brougham’s parody of the wildly popular play Metamora: Or, the Last of the Wampanoags was titled “Metamora, or the Last of the Pollywogs,” which strongly implies a pronunciation WAMP-anogs. And I just found a recording (mp3) of Chief Wild Horse, the last speaker of the Wampanoag dialect, reading the Lord’s Prayer (followed by a detailed linguistic explication) in 1961, and both he and the guy who introduces him say WAMP-anog, three syllables. So why do the dictionaries list only the spelling pronunciation wampa-NO-ag?
Addendum. Martin, in the comments, links to some extremely interesting sites: an article about Jessie “Little Doe” Fermino, a Mashpee Indian who last year earned a master’s in linguistics and is trying to revive the Wampanoag language (there’s more about the revival effort here, where the table on the upper right is, oddly, the syllabary for Inuktitut, a language not mentioned in the piece), and the website for the Wôpanâak Language Revitalization Project (and I note that the address line at the bottom refers to the “Wampanog Tribe”).
I should also mention that I got the mp3 recording from this webpage.