ESR has a theory:

"The factual piece missing from the article is that sacrificial victims were drugged into semi-consciousness so they wouldn't struggle and disrupt the ceremony.

This meant they didn't scream as obsidian knives were penetrating their chests and their hearts were ripped out to be offered to the gods. Obviously, the Aztecs considered the ceremonial experience incomplete without the screaming, and built death whistles to simulate the proper sound of agony." https://x.com/esrtweet/status/1859556171918479626