谷歌演讲文字api for php

Question

我希望将谷歌的语音用于文本API。我有以下PHP代码片段

  <?php
   $stturl = "http://www.google.com/speech-api/v2/recognize?client=chromium&lang=en-US&key=MY_KEY";
    $filename = "./good-morning-google.flac";
    $upload = file_get_contents($filename);
    //echo $upload;
    $data = array(
        "Content_Type"  =>  "audio/x-flac; rate=44100",
        "Content"       =>  $upload,
    );
    $ch = curl_init();
    curl_setopt($ch, CURLOPT_URL, $stturl);
    curl_setopt( $ch, CURLOPT_HTTPHEADER, array("Content-Type: audio/x-flac; rate=44100"));
    curl_setopt($ch, CURLOPT_POST, true);
    curl_setopt($ch, CURLOPT_POSTFIELDS, $data);
    ob_start();
    curl_exec($ch);
    curl_close($ch);
    $contents = ob_get_contents();
    ob_end_clean();
    $textarray = (json_decode($contents,true));
    var_dump($textarray);

然而，每次我调用API。呼叫成功（在谷歌开发者控制台中检查）但结果（$ contents）始终为空。有人能指出我的错误吗？

Link to .flac file