Learn Dutch: Ik hoor een stem, dus we zijn helemaal niet alleen. - I hear a voice, so we are not alone at all.

Adding helemaal (which usually means completely or entirely) right before niet amplifies the negative. While niet just means not, helemaal niet translates to not at all. It strongly emphasizes that they are definitely not alone.

Could I use helemaal geen in this sentence instead?

No, because we are negating the adjective alleen (alone), not a noun. You use geen (and therefore helemaal geen) to negate a noun. Since we are negating a state of being here, we must use niet.

Why is the word order dus we zijn instead of dus zijn we?

The word dus (so or therefore) is flexible. Here it acts as a coordinating conjunction (just like en or maar), which means it sits outside the clause and doesn't change the standard subject-verb order, giving us we zijn. However, it can also act as an adverb, which does trigger inversion (dus zijn we). Both are correct and common in Dutch!

If I wanted to say we are not alone at all anymore, where would meer go?