A long time ago (before the infantilization of the American public) this was the default, majority rule. And it's still reflected as the default position in the US Code.
There's a ton of data the federal government has that I consider proper for them to have but for not every detail to be released in the valid interests of privacy.
If you believe the public has ownership and unfettered access rights over all categories of those data, I understand your argument that these voice recordings should be no different. That's an entirely self-consistent line of reasoning.
If you think that some of that data should not be freely accessible to any member of the public, then it's a valid question to ask "do these voice recordings fall on the private or public side of that line?"
Are you sure about that? Just because it's now in the custody of the NTSB doesn't mean it's public domain. The NTSB didn't create it.