Ivona TTS works (tested with FBReader (older version - newer ones were too "appified" (stripped of settings options) with the TTS plugin), once you've enabled the Ivona apps to be able to run in the background (user settings, power management). You have to do it for both the main app, as well as all the voices app packages you have installed.
You also have to enable it for the FBReader TTS plugin (which comes as a seperate apk).
Voice samples recorded via smarphone from a bluetooth speaker (so people that havent used Ivona voices before can get an impression of how well it works):
Ivona on likebook sample - english (Amy):
https://vocaroo.com/i/s1X2h995nJ5s
Ivona on likebook sample - german (Marlene):
https://vocaroo.com/i/s0VhcPJ6ONlv
(Raised volume in audacity - because I noticed too late that recording volume was too low, so some of the minor crackle at the top end is caused by this, not the app.

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IMPORTANT: As another user in here stated before, the audio jack output quality is HORRIBLE - as in unusabe. It cuts most of the low end and some of the mid end of all audio. Bluetooth output is fine. You HAVE to the use Likebooks bluetooth audio, if you want to use it for any audio critical applications. Again - no youtube "reviewer" mentioned that even once. Thanks.
Also - if you want to get those Ivona voices going and dont have them backed up (including obb folders) from a few years ago, you are out of luck. Amazon bought the company - quickly removed the best (also free) voices they offered from Google Play - and called it a day. You might find the packages on apk mirror sites still, they are around 150mb per voice apk. You also need the corresponding Ivona Voice HQ app. And a reader that can choose TTS voices, and do so even corresponding to the ebooks language settings (FBReader (older version, using 2.5.10) can, it also has the ability to set the TTS speed most gradually, from all the TTS capable reader apps I've tested.).
Having the play store active and working probably is needed for this setup as well.
edit: All custom ttf fonts for a hacked Kindle I've aggregated over the years, work fine in the native eReader app, and all apps that use Android system fonts. Even Bookerly works. Times new roman is still better though.