Missing sound effects in level 4 of Wario Land?

I was playing through Virtual Boy Wario Land and noticed there were no sound effects during the fourth level – the boss fight against the dinosaur that spins the spiked ball. The music played fine, but all sound effects were absent. The sound effects had been playing fine in previous levels, and continued playing fine in the levels after the boss fight.

I have multiple Virtual Boys, so I tested the game on multiple systems and the problem occurred on all of them. I then thought it must be a problem with my cart, but I have two copies of the game and the same problem occurred on my second copy.

I looked up footage of that boss fight, and the sound effects play fine in all the footage I could find, but it was all emulator footage. So my question is: is there a problem with both of my copies of the game, or is this a bug with the cart in general? If it’s a problem with just my copies, any ideas with the issue could be? I’ve looked for broken traces on the PCB or anything else obvious but couldn’t spot anything.