$VoiceStyles
gives the list of available voices for speech synthesis.
Examples
Wolfram Research (2018), $VoiceStyles, Wolfram Language function, https://reference.wolfram.com/language/ref/$VoiceStyles.html.
Text
Wolfram Research (2018), $VoiceStyles, Wolfram Language function, https://reference.wolfram.com/language/ref/$VoiceStyles.html.
CMS
Wolfram Language. 2018. "$VoiceStyles." Wolfram Language & System Documentation Center. Wolfram Research. https://reference.wolfram.com/language/ref/$VoiceStyles.html.
APA
Wolfram Language. (2018). $VoiceStyles. Wolfram Language & System Documentation Center. Retrieved from https://reference.wolfram.com/language/ref/$VoiceStyles.html