WeChat.ServerMessage.XmlMessage.reply_voice

You're seeing just the function reply_voice, go back to WeChat.ServerMessage.XmlMessage module for more information.
Link to this function

reply_voice(to_openid, from_wx_no, timestamp, media_id)

回复语音消息 - 官方文档

<xml>
  <ToUserName><![CDATA[toUser]]></ToUserName>
  <FromUserName><![CDATA[fromUser]]></FromUserName>
  <CreateTime>12345678</CreateTime>
  <MsgType>voice</MsgType>
  <Voice>
    <MediaId><![CDATA[media_id]]></MediaId>
  </Voice>
</xml>