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For more information on this, take a look at:
iOS: About Siri
Can I change how Siri sounds?
New female and male voices will be available initially in U.S. English, French, and German. You can change the gender by going toSettings > General > Siri > Voice Gender. Your iOS device initially uses a compact voice for Siri. iOS will automatically download and install a more natural-sounding voice when you first connect to power and a Wi-Fi network.
Best of luck,