When minecart sounds were first introduced, they would start from a low pitch, and that pitch would gradually increase as the minecart kept moving until it reached a maximum pitch. However, from 13w42a up until the present day,the minecart sound will start off at the maximum pitch instead, which is incorrect.
Video of how the sounds used to work (expected behaviour): https://youtu.be/H418yvwzuOM
Video of how it works currently (incorrect behaviour): https://youtu.be/0Xoxuj4PU4o