It is not possible to use the default voice, and changing the voices, as well as the start and end chirps, is the principle of security and user interface design.
Changing the voice allows the user to understand that they are no longer talking to the assistant, but talking to another entity (your action). This is a crude parallel with the graphical interface, which makes it clear that you are running a user program, not one of the system programs. He also tells you that normal system commands (such as volume control) will not work.
Take the opportunity to use voice options to help set up the environment for your action and the tone that you will use to talk with the user.
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