Thanks! Simon's example uses the custom voice model (creating a voice from instructions). But that comment led me eventually to this page, which shows how to use mlx-audio for custom voices:

https://huggingface.co/mlx-community/Qwen3-TTS-12Hz-0.6B-Bas...

  uv tool install --force git+https://github.com/Blaizzy/mlx-audio.git --prerelease=allow
    
  python -m mlx_audio.tts.generate --model mlx-community/Qwen3-TTS-12Hz-0.6B-Base-bf16 --text "Hello, this is a test." --ref_audio path_to_audio.wav --ref_text "Transcript of the reference audio." --play