I don't know anything about audio, but I don't see how that would make a difference. Honestly, the only way I could see voice volume mattering would be if the microphone increased its sensitivity in order to pick up quieter voices, in which case I'd expect increased battery usage. But I don't think that happens.

Unless you're walking around saying "OK Google" and are going to start saying it too quietly for your phone to detect. :-)

Related: How does the average volume of the people you talk to affect battery life? Volume-setting being equal, will I have longer battery life if I only call whisperers?

Is this supposed to be a premature optimization joke?

Yes, if by quietly you mean so quiet you’re not talking or using the phone at all 😉

Sending a text message or email, on the whole, uses less battery.

No.