mononym I agree it'd be useful to have some TTS functionality, as I often find that having a TTS app read my document to me to be helpful for proofreading.
If you're looking for an immediate workaround, you can pass a file from Zettlr to a command line application using a custom export command, which you can set up in Preferences > Import and Export > Custom Export Commands.
As an experiment, in response to your post, I created a custom command, Speak using the command, espeak -f and when I select this export option it indeed uses the espeak command line application to speak the document open in the active tab. There are nicer command line TTS apps, like mimic and piper, as well as ones that connect to APIs of AI TTS engines. So there would seem to be a lot of possibilities.
Obviously, this isn't a perfect workaround, though. There are no playback controls, since the command is launched in the background, and if I want the command-line application to stop speaking, I have to kill the process in a terminal window (e.g., killall espeak), which is less than ideal.
One possibility would be to have the custom export command launch the a terminal window and run the command therein. For example, xterm -e espeak -f or gnome-terminal -- espeak -f. All kludges, but in the absence of a more integrated feature these could be temporary workarounds.
