Changing text-to-speech voice setting on iOS to use Enhanced Quality voice
Speaking Email uses the default system voice. In the premium edition, you can also select a voice. If you download new voices under system settings you can then select them in Speaking Email.
In Settings go to General > Accessibility > Speech > Voices.
Tap "Voice". If you don't see a "Voice" button, turn on one of the accessibility options eg "Speak Selection". Choose English (or your phone language).
You will see a number of regional voice options. Speaking Email will pick up your region from regional settings.
If your region's voice is set to "Default" it may have that Siri-with-a-cold effect. If it is, try setting it to any of the "Enhanced Quality" options. For the voice to take effect restart Speaking Email.
You can now select which voice to use in Speaking Email Premium Edition, under Preferences.