To use Whispersync for Voice on the Kindle, you need to have a Kindle with the Whispersync app activated on your Amazon account, a mobile device that has the Audible app installed, and both the Kindle and Audible versions of a book that supports Whispersync. Once all of these items are in place, synchronization between the audio and Kindle versions of the book is automatic once you start at the place you left off.
To ensure Whispersync is activated on your Amazon account, go to the Manage Your Kindle Web page, and check under Manage Your Devices to find the settings for Device Synchronization, where Whispersync should be turned on. As of 2016, the Whispersync app only works between a Kindle device and the Audible app, as both are Amazon programs. Sometimes Amazon offers discounted prices on the Audible versions of books if you already own their Kindle versions.
You don't need to purchase the Kindle and audio versions of the book simultaneously for the two versions to sync on the Whispersync app. Before purchasing both versions of the book, be sure to check that the book supports the Whispersync app by looking for the app logo in the book's information.