Which voiceId should I use in the API for a voice I can see on the website?

Fetch the voice list with the API key for your integration account and use the returned entry’s id as voiceId, not its display name on the website. Check whether its feature list includes text-to-speech or ai-cover for the task you want, and check the selected model. Being listed does not guarantee that a voice works with every model or for both uses.

GET /sound_clone/api/v1/voices