curl --request POST \
  --url https://api.opper.ai/v3/audio/speech \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/json' \
  --data '
{
  "input": "<string>",
  "model": "<string>"
}
'

{
  "audio": {
    "b64_json": "<string>",
    "file_id": "<string>",
    "mime_type": "<string>",
    "url": "<string>"
  },
  "created": 123,
  "id": "<string>",
  "model": "<string>",
  "usage": {
    "characters": 123,
    "cost": 123
  }
}