Several of these forum posts discuss how copper gears are noisy, or plastic gears break. Unfortunately, both are true, and there are no silver bullet solutions to either.
This was the compromise I came up with. I buy 30-note music box innards with the plastic gears. Then I buy copper gear replacements ( https://www.amazon.com/Wingostore-Notes-Movement-Copper-Replacement/dp/B07DG83FYS/ ). The Amazon images even show you helpful diagrams of how to take the machine apart and replace the gears. However, I only replace one gear, the one in the middle (with needle-nose pliers). That specific gear is the one that tends to break... especially if there are multiple paper strips taped together in sequence (which I do all the time). Here is what the machine looks like now: https://imgur.com/a/kuBqOE7
So now if a strip is bumpy, the plastic gear will not break. And because only one of the gears is copper, it won't be as noisy as if all gears were copper. It's a compromise, but it works for me.