PHP CURL - как составить конструкцию?

попробуйте так

$folder_id = '23232312ASdgf43';
$url_tts = 'https://pi.cloud.net/speech/tts:synthesize';
$authorization_cloud = "Authorization: Bearer " . $iam_token;
$headers[] = $authorization_cloud;
$headers[] = 'Transfer-Encoding: chunked';
$arrdata = array(
    'voice' => 'zahar',
    'emotion' => 'good',
    'format' => 'lpcm',
    'sampleRateHertz' => '48000',
    'folderId' => $folder_id,
    'text' => 'Привет мир',
    );
$ch = curl_init($url_tts);
curl_setopt($ch, CURLOPT_HTTPHEADER, $headers);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLOPT_POSTFIELDS, http_build_query($arrdata));
$result = curl_exec($ch);
curl_close($ch);

Вот так заработало.

$ch = curl_init();
$text = curl_escape($ch, $post);
curl_setopt($ch, CURLOPT_URL, $url_tts);
curl_setopt($ch, CURLOPT_HTTPHEADER, $headers);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLOPT_POST, true);
curl_setopt($ch, CURLOPT_POSTFIELDS, "voice=zahar&emotion=good&folderId=$folder_id&format=lpsm&sampleRateHertz=48000&text=$text");
$result = curl_exec($ch);