Now i had to brush up the course. I am now wondering this too. The word is preceeding by "nhiều", so it should have the classifier.
I ended up asking this from a native how would] she say this. She said she would omit the classifier, because she is lazy. You don't need the classifier, because everyone knows "voi" means the elephant.
I hear nothing wrong with that either, unless you meant the difference between the north and the south accents, which the latter's nhiều voi might sound [ɲiw˨˩ jɔj˧˧] unlike the former's [ɲiəw˨˩ vɔj˧˧]. both are correct. you know you are talking about a native speaker reading in his own language, right?