No, my translation is " What's the origin of this?". So, "Откуда пахнет?" "Откуда идет запах?" - is this correct?

I don't know. Let's say you're sitting with your friend and you both smell smoke. You look at her and say "Where's that coming from?" Alternatively, you hear screams and ask "Where is that coming from?"

Then simply "Откуда это?" ( it may refer to the smell or voice or music etc.). Right?

Александр Попов translated correctly

That is correct

+Alexander