How do I get Actions on Google to speak using Google Home's normal voice?

There is no way to use the "default" voice, and the change in voices as well as the starting and ending chirp are both a security and a UI design principle.

The change in voice lets the user know they are no longer talking to the Assistant, but are talking to another entity (your Action). This is a rough parallel to a GUI that makes it clear you're running a user program and not one of the system programs. It also cues you that the usual system commands (adjusting volume, for example) won't work.

Take the opportunity to use the voice options to help set the environment for your Action and the tone that you'll be using to talk to the user.