Question

When would the velocity of a wave traveling in a medium change ?

Answer 1

Occurs when its going from one medium to another. But remember when the density of the medium is changed, the velocity of the wave would most likely change.

Answer 2

The velocity of a wave travelling in a medium changes when the medium of the wave changes

Factors affect the velocity would be the refractive index of the medium

Higher the refractive index denser will be the medium and lower will be the velocity.

And vice-versa that is lower the refractive index rarer will be the medium and higher will be the velocity.