Using `say` to create my own audiobooks
Optional Voices in OS X

OS X includes support for numerous languages, take a look at the Other Languages section of Speaking with a French accent – OS X say. The optional voices include alternative English speakers:


  To discover the full list of voices and associated languages:
  
  
  Open System Preferences.app
  Select Dictation &amp; Speech
  Select System Voice > Customize…
  




Customising Synthesised Speech

Apple's development documentation includes Techniques for Customizing Synthesized Speech. This is a technical document aimed at developers but it includes a section on Use Embedded Speech Commands to Fine-Tune Spoken Output that offers a way to embed additional statements within your text that influence the synthesised voice.

Cepstral: Buy Professional Voices

You can also purchase additional voices for use with OS X. Voices are available from Cepstral:


  CereVoice text-to-speech is available for Apple Mac OS X, bringing CereProc's high-quality voices to computers running Apple's OS X: 10.5 Leopard, 10.6 Snow Leopard, 10.7 Lion, 10.8 Mountain Lion, 10.9 Mavericks, 10.10 Yosemite and 10.11 El Capitan (Intel Macs only). CereVoice can replace the default Mac voices with a wide range of other accents and languages.