GoKawiiliOS 27 beta 5 has further expanded the new Siri voice customization options. In earlier betas, the new Pace and Expressivity sliders were only available for the two newly-introduced Siri voices that have an American accent.
But now, the customization is available for any American voice, and all of the existing British accent voices as well …
That means there are now eleven supported voices which allow for choosing different pace (speed of talking) and expressivity (the amount of emotion and emphasis) settings.
You can customize Pace and Expressivity by going to Settings -> Siri -> Voice and dragging the sliders. There are five supported levels for both pace and expressivity.
On a modern iPhone, that means there are a total of eight American voice options and four British voices to pick from, that support pace and expressivity adjustments.
Apple previously indicated that the new Siri voices take advantage of a new powerful on-device model, but it has pretty recent hardware requirements. The feature requires an iPhone 17 Pro, iPhone 17 Air, iPad Pro (M4) and newer, and Macs with M3 chip or newer and at least 12 GB RAM.
Siri AI itself requires an iPhone 15 Pro or newer; you just get the older voice synthesis on older hardware. Siri AI will be launching in beta sometime later this year.