Please use the List Your Cloned Voices endpoint instead.

If you have cloned voices, the endpoint will return a JSON containing information about them. You can use these voices by setting the returned value as the voice parameter in the Generate Audio endpoint, for example: s3://mockingbird-prod/william_3_d2b62fd7-a52d-4bd1-a09a-fb2748eda979/voices/william_3/manifest.json.

Click Try It! to start a request and see the response here!