如何从Dialogflow CX webhook发送回音频？

const express = require('express');
const app = express();

app.post('/webhook', (req, res) => {
  // 处理来自CX Agent的请求
  const userQuery = req.body.query;
  
  // 调用语音合成服务生成音频文件
  const audioUrl = generateAudio(userQuery);
  
  // 构建回复消息
  const response = {
    fulfillment_response: {
      messages: [
        {
          platform: 'TELEPHONY',
          telephony_synthesize_speech: {
            ssml: `<speak><audio src="${audioUrl}"/></speak>`
          }
        }
      ]
    }
  };
  
  // 返回回复消息给CX Agent
  res.json(response);
});

app.listen(3000, () => {
  console.log('Webhook服务已启动，监听端口3000');
});