Standard pronounciation has changed quite a bit over the centuries, but old rhymes are often retained in poetic diction long after they have been dropped in normal usage ("find" and "wind", for example). In this case, though, the words will have rhymed quite comfortably when they were written; in the 18th century, for example, "tea" was generally pronouced "tay" (approximately), and I'd think that much the same would be true of "speak" in this case. The older pronounciation persists in a number of regional dialects of English.