Well brains size is a factor, but isn't the only factor in determining intelligence. We don't really have a very accurate way of measuring intelligence in live subjects, much less long dead fossils. Though I've heard it said that Neandertals were smart enough to master complex tasks such as flying a commercial airliner, but we have know way of knowing if that is true, or if the neandertals would have produced an Albert Einstein or a Steven Hawking if they were not supplanted by homo sapiens.
There is no such thing as being better suited for survival, only better suited for survival in a specific environment. Neadertals were better adapted for survival in the Ice age European north. Homo sapiens were well adapted to life in the Fertile cresent, where high protein cereal grains like wheat and barley and animal domestication allowed a surplus of food to be created and stored, allowing the freedom for some individuals to persue other projects such as more advanced building materials and metal working.
An undecended larynx does not mean it is more difficult or requires more effort to speak, only that the pitch may have been higher, or maybe not as many animals including wolves and elk have larynxes that can decend deeper in their throats when the need to make deeper noises. Higher pitches are often easier to pick out from natural background noises which are usually much lower in tone, but don't carry as far. For another thread on a similar topic see Origin of speech in Klingons