We've built our own text-to-speech system with an initial English language model we trained ourselves with fully open source data. It will be added to our App Store soon and then included in GrapheneOS as a default enabled TTS backend once some more improvements are made to it.
We're going to build our own speech-to-text implementation to go along with this too. We're starting with an English model for both but we can add other languages which have high quality training data available. English and Mandarin have by far the most training data available.
Existing implementations of text-to-speech and speech-to-text didn't meet our functionality or usability requirements. We want at least very high quality, low latency and robust implementations of both for English included in the OS. It will help make GrapheneOS more accessible.
@GrapheneOS What about SherpaTTS? It is of high-quality, support a wide range of languages, and is libre software.