There must be air molecules for sound to travel.
There must be air molecules for sound to travel on.
Which is better, "travel" or "travel on"?
There must be air molecules for sound to travel.
There must be air molecules for sound to travel on.
Which is better, "travel" or "travel on"?
Travel would be better, as on is not a correct preposition here. Sound travels 'through' the air. An analogy is to say we drove through, the countryside in a car. The country side maintains its position, the car does not.
Also ote the distinction between the boat travels on the water, the submarine travels through the water. The former is on top of the water, the later is surrounded by it. Thus you could say sound travels in air, but not on it.
The on/in distinction can be confusing when talking about travel. You can say travel in a car, but not on a car, but you can say travel on a bus, plane, train etc. In the latter case though we say in First Class, in the Carriage when referring to the part used for traveling. We use on when referring to the whole where something that has a large enough floor area to stand on, (working as a contraction of upon), and where we distinguish between travel and location, hence on the train but in the carriage. The distinction can also referrer to (en)closed/open: in a car but on a motorbike. Its idiomatic, though, you can say sat 'on' the top deck of a double decker bus, even though it has less room than a train carriage. That's because the term deck comes from ships where they are always open.