当我发送以下有效负载时，我得到了包含正确内容的200 OK：

<speak version='1.0' xmlns='http://www.w3.org/2001/10/synthesis' xml:lang='en-US'><voice name='Microsoft Server Speech Text to Speech Voice (fi-FI, HeidiRUS)'>This is my test</voice></speak>

下面是我的C#代码发送这个：

// Generate request
string body = $@"<speak version='1.0' xmlns='http://www.w3.org/2001/10/synthesis' xml:lang='{voiceLang}'><voice name='{voiceName}'>{text}</voice></speak>";

using (var client = new HttpClient())
{
    using (var request = new HttpRequestMessage())
    {
        // Set the HTTP method
        request.Method = HttpMethod.Post;
        // Construct the URI
        request.RequestUri = new Uri(ttsHostUri);
        // Set the content type header
        request.Content = new StringContent(body, Encoding.UTF8, "application/ssml+xml");
        // Set additional header, such as Authorization and User-Agent
        request.Headers.Add("Authorization", "Bearer " + accessToken);
        request.Headers.Add("Connection", "Keep-Alive");
        // Update your resource name
        request.Headers.Add("User-Agent", "YOUR_RESOURCE_NAME");
        request.Headers.Add("X-Microsoft-OutputFormat", "riff-24khz-16bit-mono-pcm");
        // Create a request
        using (var response = await client.SendAsync(request).ConfigureAwait(false))
        {
            response.EnsureSuccessStatusCode();

            // Asynchronously read the response
            using (var dataStream = await response.Content.ReadAsStreamAsync().ConfigureAwait(false))
            {
                // ... Process your audio here
            }
        }
    }
}

我在我们的负载上看到的唯一区别是xmlns值，其中我有一个完整的url（带有http：//），而你的没有。
错误可能在其他地方：当查询端点时你确信你正在验证你自己吗？

我们还突然收到了400错误，在我们的示例中，我们没有使用
<![CDATA[ your text here ]]>
在我们的代码中这样做之后，我们不再得到用户报告的400个错误。