Create an object of the loudnessMeter System object™, and then call visualize to open an 'EBU Mode' loudness meter display.

loudMtr = loudnessMeter;
visualize(loudMtr)

Create an audio file reader System object and specify the audio file to analyze. Create an audio device writer System object to play the audio to your output device.

fileReader = dsp.AudioFileReader('RockDrums-48-stereo-11secs.mp3');
deviceWriter = audioDeviceWriter('SampleRate',fileReader.SampleRate);

In an audio stream loop, read the audio from the file and play it to your device. The loudness meter visualization updates at each call.

while ~isDone(fileReader)
    audioIn = fileReader();
    loudMtr(audioIn);
    deviceWriter(audioIn);
end