"Ngày mai là thứ tư?"
Translation:Is tomorrow Wednesday?
Like in English, there are points of inflection in Vietnamese to indictae a question. Too bad I can't tell you the difference over an audio file. I can assure you that this speaker did have a point of inflection on the very last word. It's so subtle, even for a tonal language.
I put "Is it Wednesday tomorrow?" and the answer said "It is Wednesday tomorrow?". If the second one is correct, the first one definitely should be! Because, while you CAN make it a question with just your inflection, the normal way would be with the word order, "Is it..."