Siri voice unavailable
Apple does not allow 3rd party apps to use the Siri voices. Speaking Email will however use any 'Enhanced' voices that you have available on your phone. You can check and download extra 'Enhanced' voices under System Settings > General > Accessibility > Voices.
To select a voice to use in Speaking Email, go into Preferences > Voices and choose the voice you like for each language. Note there are two 'English' selections - one for the 'announcer' and one for English email reading.