From what a local told me, the car horns in Mumbai are by design less loud than the ones in Europe or the US. This fits my experience, but that might also be simply being used to the sound.
Many people change the horn from the factory fitted one to another of their liking (louder and/or different sound pattern/music), though it’s not legal. I don’t believe that the factory fitted horns are less loud than in Europe or the US.