Nós = we
Nos = us (Você nos deu o vinho./ You gave us the wine)
Nosso = ours/our (Você bebeu nosse vinho./ You drank our wine.)
I am very confused about Nos and Nós, hope it gets clear in later levels.
Why the accent in this case?
The subject pronoun "nós" always have an accent. You may be confusing it with the object pronoun "nos". It's similar (with a few exceptions) to we/us in English.
Why does the voice pronounce nh like it was only n? Is h mute in this case, like it was written 'vino'?
It is similar to "lasagna" in English.