Reviving an old thread here.
From the posts above I noticed a common complaint about the robotic-sounding voices from ESpeak and RHVoice, which I have to agree with. Even when I used Google Speech Services, I found the voice quality to be subpar.
Since then, I’ve used Sherpa TTS, and the voice quality is excellent, at least with the voice model I downloaded. It works well with Organic Maps, as mentioned in the thread above (MoonshineMidnight from Dec 9, 2023).
Links to F-Droid app site and Github pages below.
https://f-droid.org/packages/org.woheller69.ttsengine/
https://k2-fsa.github.io/sherpa/