Hi @JamesXanda, you can export it with the CLI as follows:
optimum-cli export onnx --model microsoft/speecht5_tts test_onnx --model-kwargs '{"vocoder": "microsoft/speecht5_hifigan"}'
Let me know if that works on your side.
Note that you can use the latest stable release of Optimum for exporting this model, no need to install it from source.