How can I add voices on my iPhone
Speaking Email uses the Apple voices installed on your iPhone. You can easily download more enhanced quality voices.
In iPhone Settings, select General > Accessibility > Speech > Voices
There you can download voices for any language. You need to be on wifi to download voices since they are quite big files.
Back in Speaking Email you can now select those voices you've just downloaded.
When you upgrade to Premium, you can choose any voice to be your email reader, and a second voice to be your "announcer".