Piper is a VAE model which is quite robotic. This is a speech language model, which sound quite realistic.
You can listen to the model on this video => https://www.youtube.com/watch?v=YAB3hCtu5wE
The codec is open source: https://huggingface.co/neuphonic/neucodec
Then may I suggest that this should be edited?
> Audio Codec: NeuCodec - our proprietary neural audio codec that achieves exceptional audio quality at low bitrates using a single codebook
( https://huggingface.co/neuphonic/neutts-air#model-details )
> The codec is open source: https://huggingface.co/neuphonic/neucodec
This says it was trained on proprietary data.