首先分析下要解决的问题：

    1.获取媒体播放的音量。

    2.通过seekbar可以增减音量

    3.用户按下音量键增减音量，seekbar保持同步

对于第一个问题：Android系统提供AudioManager类来获得系统audio服务。

对于第二个问题：实现seekBarChangeLIstener里面onProgressChanged方法。

对于第三个问题：用线程来同步更新UI。

设置音量的代码：

 private void setVolum()
 {
    maxVolume = mAudioManager.getStreamMaxVolume(AudioManager.STREAM_MUSIC);
    volSeekBar.setMax(maxVolume);
    currentVolume = mAudioManager.getStreamVolume(AudioManager.STREAM_MUSIC);
    volSeekBar.setProgress(currentVolume);
    mVolume.setText(currentVolume*100/maxVolume + " ");
 }

OnSeekBarChangeListener seekBarChangeListener = new OnSeekBarChangeListener()
{
    public void onProgressChanged(SeekBar seekBar, int progress,
    boolean fromUser) {
       // TODO Auto-generated method stub
       mAudioManager.setStreamVolume(AudioManager.STREAM_MUSIC, progress, 0);
    }

    public void onStartTrackingTouch(SeekBar seekBar) {
       // TODO Auto-generated method stub

    }

    public void onStopTrackingTouch(SeekBar seekBar) {
       // TODO Auto-generated method stub

    }

};

 Handler myHandler = new Handler(){
          public void handleMessage(Message msg) {  
              switch (msg.what) {  
              case PROGRESS_CHANGED:  
                  setVolum();  
                  /*sendEmptyMessageDelayed(PROGRESS_CHANGED, 200);*/
                  break;
             }
          }
     };
 
  class myVolThread implements Runnable {   
         public void run() {  
              while (!Thread.currentThread().isInterrupted()) {    
                      
                   Message message = new Message();   
                   message.what =PROGRESS_CHANGED;   
                   MainActivity.this.myHandler.sendMessage(message);   
                   try {   
                        Thread.sleep(100);    
                   } catch (InterruptedException e) {   
                        Thread.currentThread().interrupt();   
                   }   
              }   
         }   
    }   

public class MainActivity extends Activity {
	
	protected static final int PROGRESS_CHANGED = 0x101;
	private SeekBar volSeekBar ;
	private TextView mVolume ;
	private AudioManager mAudioManager;
	private int maxVolume, currentVolume;
	
	Thread myVolThread = null;
	Handler myHandler = new Handler(){
    	public void handleMessage(Message msg) {  
            switch (msg.what) {  
            case PROGRESS_CHANGED:  
            	setVolum();  
            	/*sendEmptyMessageDelayed(PROGRESS_CHANGED, 200);*/
            	break;
           }
        }
    };
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        
        volSeekBar= (SeekBar)findViewById(R.id.volum_seek);
        mVolume = (TextView)findViewById(R.id.volum_val);
        mAudioManager = (AudioManager)getSystemService(Context.AUDIO_SERVICE);
        
        //setVolum();
        OnSeekBarChangeListener seekBarChangeListener = new OnSeekBarChangeListener()
        {

			public void onProgressChanged(SeekBar seekBar, int progress,
					boolean fromUser) {
				// TODO Auto-generated method stub
				mAudioManager.setStreamVolume(AudioManager.STREAM_MUSIC, progress, 0);
			}

			public void onStartTrackingTouch(SeekBar seekBar) {
				// TODO Auto-generated method stub
				
			}

			public void onStopTrackingTouch(SeekBar seekBar) {
				// TODO Auto-generated method stub
				
			}
        	
        };
		
        volSeekBar.setOnSeekBarChangeListener(seekBarChangeListener);
        new Thread(new myVolThread()).start();
        
    }
    private void setVolum()
    {
    	
    	maxVolume = mAudioManager.getStreamMaxVolume(AudioManager.STREAM_MUSIC);
        volSeekBar.setMax(maxVolume);
        currentVolume = mAudioManager.getStreamVolume(AudioManager.STREAM_MUSIC);
        volSeekBar.setProgress(currentVolume);
        mVolume.setText(currentVolume*100/maxVolume + " ");
    }
    
    class myVolThread implements Runnable {   
        public void run() {  
             while (!Thread.currentThread().isInterrupted()) {    
                     
                  Message message = new Message();   
                  message.what =PROGRESS_CHANGED;   
                  MainActivity.this.myHandler.sendMessage(message);   
                  try {   
                       Thread.sleep(100);    
                  } catch (InterruptedException e) {   
                       Thread.currentThread().interrupt();   
                  }   
             }   
        }   
   }   
}