honestly, the options within the Kindle ecosystem itself are... limited. Kindle's built-in VoiceView screen reader is functional but robotic. It gets the job done if you just need the words read aloud, but for any kind of sustained listening, that monotone drone is a real mood killer. It feels like an accessibility tool, not a pleasure.
That's why I look outside the Kindle app. On my phone, I use the Kindle app paired with my device's native text-to-speech engine, which on recent Android and iOS versions is surprisingly decent. The voices have better intonation. But the real game-changer for me was using an audiobook subscription service like Audible alongside my Kindle purchases. If a Whispersync for Voice title is available, it's seamless—my progress syncs, and I get a professional human narration.
For actual TTS apps that can handle sideloaded EPUBs or PDFs, @Voice Aloud Reader on Android has a ton of natural-sounding cloud-based voices you can download. The process is clunky, though: send the book to the app, hope formatting is okay. It's a patchwork solution. I mostly just wish Amazon would invest in a premium, natural-sounding TTS option as an in-app purchase for books without audiobooks.