Do you mean to turn on TTS? If it's enabled for the book (it isn't necessarily), you'll find it under 'font' options. While the book is open, tap the page to get the top menu and tap the Aa Settings option, first from the left. Tap More Options in the bottom of the menu that pops up. Text to Speach wil show there as well as a notation that indicates whether it's available for the title.
If what you want to do is listen to the audible file while reading the kindle file, someone else will have to help you with that. I know you need to first have both files, but I don't really do audio so am not sure of the steps. I would expect it to be in the users guide, however, which is accessible by swiping down from the top to show the settings menu. Tap more and the top option is Help and Feedback. You do need to be connected via wireless to access info there.
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