I'm not sure whether this is intended behaviour, but minecarts are permanently silenced after they cross rails on wool, even if they move onto rails on other types of blocks afterwards (unless they're picked up and placed back down again).

The silencing effect also only works when the player is not in the minecart - when I get into the minecart in first person view, the minecart is noisy again, even on wool.

Duplicate of #4464