The list of voices returned by VoiceStyleData and $VoiceStyles should be the same on the same machine. They vary depending on your platform. We use some generic voices like "AlanWBlack" that are available on all platforms, then on Windows and Mac, we do use operating system voices that are platform specific.
Speak respects system settings. For example, on mac, you can change the voice in System Preferences > Accessibility > Speech. Default voice in SpeechSynthesize does not currently respect system settings.
There are obviously some consistency updates and additional utilities and voice properties that we can introduce here.
