How do I get the current volume/amplitude in a MediaPlayer?

You are in luck. There is a class called Visualizer which will do what you want I think.

import android.app.Activity;
import android.media.audiofx.Visualizer;
import android.os.Bundle;
import android.util.Log;


public class MainActivity extends Activity {
private Visualizer audioOutput = null;
public float intensity = 0; //intensity is a value between 0 and 1. The intensity in this case is the system output volume
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    createVisualizer();

}


private void createVisualizer(){
    int rate = Visualizer.getMaxCaptureRate();
    audioOutput = new Visualizer(0); // get output audio stream
    audioOutput.setDataCaptureListener(new Visualizer.OnDataCaptureListener() {
        @Override
        public void onWaveFormDataCapture(Visualizer visualizer, byte[] waveform, int samplingRate) {
            intensity = ((float) waveform[0] + 128f) / 256;
            Log.d("vis", String.valueOf(intensity));
        }

        @Override
        public void onFftDataCapture(Visualizer visualizer, byte[] fft, int samplingRate) {

        }
    },rate , true, false); // waveform not freq data
    Log.d("rate", String.valueOf(Visualizer.getMaxCaptureRate()));
    audioOutput.setEnabled(true);
}


}

you will need these permissions:

<uses-permission android:name="android.permission.RECORD_AUDIO"></uses-permission>
<uses-permission android:name="android.permission.MODIFY_AUDIO_SETTINGS"></uses-permission>

You can get the current volume of media player with the help of Audiomanager class.The code is as follows:-

AudioManager am = (AudioManager) getSystemService(AUDIO_SERVICE);
int volume_level= am.getStreamVolume(AudioManager.STREAM_MUSIC);

Similarly,if you want to set the default volume of media player.You can do that like as:-

am.setStreamVolume(
            AudioManager.STREAM_MUSIC,
            volume_level,
            0);

That's all..Happy coding :)