@Rozay101Wetter I think I need to point out a little fallacy here: Being a native speaker doesn't mean someone is an expert in all fields of their language. Paulenrique is a native speaker, yes. I'm sure he knows a lot about his language, but what he said was simply wrong.
Now, of course, I am going to provide you with a source: http://en.wiktionary.org/wiki/av%C3%B4
Vô is simply a shortening of avô, so the o-sound is the same. Now, if you look, there's a section about the pronunciation. It says there, that in Brazil, this word is generally pronounced /aˈvo/. I will assume that you can't read IPA (if you can, you don't need to read on, everything should be clear now).
Paulenrique was talking about a nasal vowel. In IPA (the stuff between those slashes), nasal vowels are marked with a tilde ~ above it. So if avô (and therefore also vô) had one, it would be written /aˈvõ/. The exact same symbol is also used in Portuguese writing: coração (heart) - corações. Here you have a nasal vowel.
Nowhere else in any other Duolingo course did I see where the omission of an accent literally could change the meaning of a word. May be that there is... I just don't recall one and I've been around. That said-- I agree with your point-- for beginners especially, demanding every accent be correct makes it quite a cumbersome task.