When armor stands are crammed into a confined minecart, no sound will be made if armor stands are summoned before the minecart
Only armor stands summoned after the minecart will cause the minecart to sound.
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I cannot reproduce the minecart making noise when the armor stand(s) are dropped on top of it from my testing. Can you please attach the world you've tested this in?
After testing in the attached world, I observed that this behavior is not exclusive to armor stands, and I would argue duplicates MC-100180 as fixing that issue would also fix this (no sound would play at all, as the minecart is stationary)