Thank you. I spoke with Apple Support, and they were unable (so far) to restore Siri’s American female voice. Downloading alternate language voices, or switching to the American male voice worked as well.
I am assuming that this means the American female voice file somehow got deleted or corrupted. It seems odd that you cannot just download it again as you can for alternate languages. I did download several alternates (to see which I preferred) and noticed that after a while, the ones that I did set permanently were automatically deleted. I discovered this by changing the language again, and it prompted me to download again.
Apple later responded to an earlier Tweet advising that this is a known issue and that they do not have a fix yet. They advised to keep an eye open for new iOS updates:
“Thanks for reaching out. We’re happy to help. We’ve heard reports of the issue you’re describing, and it’s currently under review. We don’t have a solution at this time, but recommend that you keep your software up-to-date.
As a workaround, as you’ve noticed, change Settings > Siri & Search > Siri Voice to a different Siri voice to test. You can then use the alternate voice temporarily.”
Thanks again for your assistance!