403(禁止)，Google Speech API上的密钥错误无效

我试图使用以下url访问语音api，但总是得到403（禁止）错误，无效密钥。

https://www.google.com/speech-api/v2/recognize?output=json&lang=en-us&密钥＝｛MyKey｝

我已经尝试使用服务器密钥和浏览器密钥，如此密钥图像所示。

我正在使用.NET http客户端发送http请求；代码片段如下：

Stream stream = null;
            StreamReader sr = null;
            WebResponse response = null;
            JSon.RecognizedItem result;
            try
            {
                WebRequest request = WebRequest.Create(Constants.GoogleRequestString);
                request.Method = "POST";
                request.ContentType = "audio/x-flac; rate=" + sampleRate;
                request.ContentLength = bytes.Length;
                stream = request.GetRequestStream();
                stream.Write(bytes, 0, bytes.Length);
                stream.Close();
                response = request.GetResponse();
                stream = response.GetResponseStream();
                if (stream == null)
                {
                    throw new Exception("Can't get a response from server. Response stream is null.");
                }
                sr = new StreamReader(stream);
                //Get response in JSON format
                string respFromServer = sr.ReadToEnd();
                var parsedResult = JSon.Parse(respFromServer);
                result =
                    parsedResult.hypotheses.Where(d => d.confidence == parsedResult.hypotheses.Max(p => p.confidence)).FirstOrDefault();
            }
            finally
            {
                if (stream != null)
                    stream.Close();
                if (sr != null)
                    sr.Close();
                if (response != null)
                    response.Close();
            }
            return result == null ? "" : result.utterance;

有人能帮我找出这里的问题吗。

谢谢，Ab.