I was kind of desperate for better audio for Korean than the Siri voice or the other one that's available through the app and found this site..
https://ttsmp3.com
It says it's free so I thought maybe it's something the app could download with the touch of a button? The site has a built in feature to download MP3 files so I've been doing that over and over and over. It's worth it but obviously if the app could download from here with the ease that you can download using the other TTS voices it would be wonderful.
Either way, if you were to add more TTS options, even if they required a bit of money, I'd be very interested. Good audio helps me focus a lot more.