If several minecarts are lined up on a slope and started at the same time, there will be no sound from some of the minecarts.
The minecarts that lose sound change randomly each time.
This anomaly does not occur if only one minecart is used to start the train.
Linked issues
duplicates 1
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Probably the same root cause as MCPE-113484.
This is probably caused by the game's optimization code not loading additional sounds when it starts to run out of memory space or processing time in a single tick. For that reason I expect is is working as intended.