code: func (builder *SSMLTextBuilder) AppendAudio(src string) *SSMLTextBuilder {
  
  	builder.buffer.WriteString(fmt.Sprintf("<audio src=\"%s\"/>", src))
  
  	return builder
  }
docs: // AppendAudio will append the playback of an MP3 file to the response. The audio playback
  // will take place at the specific point in the text to speech response.