code: func (m *AudioMessage) MarshalJSON() ([]byte, error) {
  	return json.Marshal(&struct {
  		Type               MessageType      `json:"type"`
  		OriginalContentURL string           `json:"originalContentUrl"`
  		Duration           int              `json:"duration"`
  		QuickReply         *QuickReplyItems `json:"quickReply,omitempty"`
  	}{
  		Type:               MessageTypeAudio,
  		OriginalContentURL: m.OriginalContentURL,
  		Duration:           m.Duration,
  		QuickReply:         m.quickReplyitems,
  	})
  }
docs: // MarshalJSON method of AudioMessage