with sound it can go the other way - a more dense medium can transmit sound more easily, and the sound will travel faster.
The speed of light is a constant - but only in a vaccuum. Like the speed of sound is only 340 m/s in dry air at 20 degrees C. It varies with the medium.
I do see you point though - it would be nice to know the exact nature of the interaction with the medium that causes the change in speed. With sound it is almost self explanatory - with light - less so.
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