If you select "Batch Download" when you first download the game after starting gameplay, you can select either "Voices On" or "Voices Off" at the beginning of a chapter. However, if you select "Voices Off" here, you cannot change to "Voices On" in the middle of the chapter.
When you move on to the next chapter, you will be able to select either "Voices On" or "Voices Off" again, so please change it to "Voices On" there.
If you want to play with voices in the middle of a chapter, you can download all game data including voices by selecting "Download All Data" from the title screen menu. This will increase the size of the app, so please consider the available space on your device before doing so.