Why is there no postulate about the constancy of the speed of sound?

The speed of a wave depends on the density of the medium in which it propagates, and the higher the density, the greater the speed. From here it follows that the light also spreads in a certain medium with a huge density (ether). Due to the imperfection of measurement methods, the theory of ether was rejected and it was decided that the speed of light is constant and does not change. This is how the theory of relativity appeared.

Remember: The process of learning a person lasts a lifetime. The value of the same knowledge for different people may be different, it is determined by their individual characteristics and needs. Therefore, knowledge is always needed at any age and position.