What is the purpose of this tune playing? To warn the buffaloes, sorry people (maybe buffaloes and cows occasionally) that are behind the vehicle to move out of the way. You might think that that is something that would logically happen when one sees a car or truck coming at them. But no, not in India. Be it a person or animal, they continue with whatever they were doing ignoring the vehicle until it almost bumps into them and then (this is even better) glare at the driver as though the fault were his/hers. For a passing observer though, this tune playing is so annoying because it does not cease until the reverse gear is disengaged and that usually happens only after the confrontation with the party blocking the reverse has ended; a quite laborious process.
If you're ever in a state of amnesia and have forgotten which city or state in India you are in, you can find out based on the accent of the person that says the most famous of these reverse tunes "Attention please! This car is backing up"! "Attention please! This car is backing up"! "Attention please! This car is backing up"!
It makes me want to screeeeeeeeeeeeeeam!!!!!!!!!!!!!!!!!!