Text to speech: pitch value is not considered

I am making calls in php to the rest api of google text to speech.

I make the call in this way:

    $url = "https://texttospeech.googleapis.com/v1/text:synthesize?key=".$key;

    $data = array(
                    'input' => array(
                        'ssml' => $testo
                    ),
                    'voice' => array(
                        "languageCode" => "en-us",
                        "name" => "en-US-Wavenet-I",
                        "ssmlGender" => "MALE"
                    ),
                    'audioConfig' => array(
                        'audioEncoding' => "LINEAR16",
                        "effectsProfileId" =>  [
                          "small-bluetooth-speaker-class-device"
                        ],
                        "speakingRate" => 1,
                        "pitch" => 4,
                     )
              );

    $options = array(
        'http' => array(
                    'header' => "Content-Type: application/json\r\n",
                    'method' => 'POST',
                    'content' => json_encode($data)
                )
            );

            $context = stream_context_create($options);
            $response = file_get_contents($url, false, $context);
          
            if ($response === false) {
                
            } else {
            
                $response_data = json_decode($response, true);
                ......
            }

The response seems to disregard the pitch value. I have tried both passing it as an integer and as a string but the result does not change. What am I doing wrong?

Thank you.

Should anyone have the same problem; I have found a possible solution:

I had $testo = '<speak> text I need audio </speak>';

I added an internal prosody tag to speak:

$testo = '<speak><prosody pitch="'.$pitchValue.'st"> text I need audio </prosody ></speak>';

I obtain the desired result.