Returns a list of ``Voice`` supported for synthesis.

        Example:
            >>> from google.cloud import texttospeech_v1beta1
            >>>
            >>> client = texttospeech_v1beta1.TextToSpeechClient()
            >>>
            >>> response = client.list_voices()

        Args:
            language_code (str): Optional (but recommended)
                `BCP-47 <https://www.rfc-editor.org/rfc/bcp/bcp47.txt>`__ language tag.
                If specified, the ListVoices call will only return voices that can be
                used to synthesize this language\_code. E.g. when specifying "en-NZ",
                you will get supported "en-*" voices; when specifying "no", you will get
                supported "no-*" (Norwegian) and "nb-*" (Norwegian Bokmal) voices;
                specifying "zh" will also get supported "cmn-*" voices; specifying
                "zh-hk" will also get supported "yue-\*" voices.
            retry (Optional[google.api_core.retry.Retry]):  A retry object used
                to retry requests. If ``None`` is specified, requests will not
                be retried.
            timeout (Optional[float]): The amount of time, in seconds, to wait
                for the request to complete. Note that if ``retry`` is
                specified, the timeout applies to each individual attempt.
            metadata (Optional[Sequence[Tuple[str, str]]]): Additional metadata
                that is provided to the method.

        Returns:
            A :class:`~google.cloud.texttospeech_v1beta1.types.ListVoicesResponse` instance.

        Raises:
            google.api_core.exceptions.GoogleAPICallError: If the request
                    failed for any reason.
            google.api_core.exceptions.RetryError: If the request failed due
                    to a retryable error and retry attempts failed.
            ValueError: If the parameters are invalid.

def list_voices(
        self,
        language_code=None,
        retry=google.api_core.gapic_v1.method.DEFAULT,
        timeout=google.api_core.gapic_v1.method.DEFAULT,
        metadata=None,
    ):
        """
        Returns a list of ``Voice`` supported for synthesis.

        Example:
            >>> from google.cloud import texttospeech_v1beta1
            >>>
            >>> client = texttospeech_v1beta1.TextToSpeechClient()
            >>>
            >>> response = client.list_voices()

        Args:
            language_code (str): Optional (but recommended)
                `BCP-47 <https://www.rfc-editor.org/rfc/bcp/bcp47.txt>`__ language tag.
                If specified, the ListVoices call will only return voices that can be
                used to synthesize this language\_code. E.g. when specifying "en-NZ",
                you will get supported "en-*" voices; when specifying "no", you will get
                supported "no-*" (Norwegian) and "nb-*" (Norwegian Bokmal) voices;
                specifying "zh" will also get supported "cmn-*" voices; specifying
                "zh-hk" will also get supported "yue-\*" voices.
            retry (Optional[google.api_core.retry.Retry]):  A retry object used
                to retry requests. If ``None`` is specified, requests will not
                be retried.
            timeout (Optional[float]): The amount of time, in seconds, to wait
                for the request to complete. Note that if ``retry`` is
                specified, the timeout applies to each individual attempt.
            metadata (Optional[Sequence[Tuple[str, str]]]): Additional metadata
                that is provided to the method.

        Returns:
            A :class:`~google.cloud.texttospeech_v1beta1.types.ListVoicesResponse` instance.

        Raises:
            google.api_core.exceptions.GoogleAPICallError: If the request
                    failed for any reason.
            google.api_core.exceptions.RetryError: If the request failed due
                    to a retryable error and retry attempts failed.
            ValueError: If the parameters are invalid.
        """
        # Wrap the transport method to add retry and timeout logic.
        if "list_voices" not in self._inner_api_calls:
            self._inner_api_calls[
                "list_voices"
            ] = google.api_core.gapic_v1.method.wrap_method(
                self.transport.list_voices,
                default_retry=self._method_configs["ListVoices"].retry,
                default_timeout=self._method_configs["ListVoices"].timeout,
                client_info=self._client_info,
            )

        request = cloud_tts_pb2.ListVoicesRequest(language_code=language_code)
        return self._inner_api_calls["list_voices"](
            request, retry=retry, timeout=timeout, metadata=metadata
        )