how can i retrieve a random value Firebase database

I have a firebase database with a tree that looks like this

tvthemetunes
airwolf value "url to air wolftheme"
eastenders value "url to eastenders"
knight rider value "url to nightrider"

and so on…

in my app users download items and while items download a tv theme will play from the url. i can get it to work ok when value event for a single item. i want it to pick a value at random from the list. how can achieve this?

this is my code for single item

 protected Dialog onCreateDialog(int id) {

    switch (id) {
        case progress_bar_type:
//Here is where i play the theme
            getthemetune();
            pDialog = new ProgressDialog(c);
            pDialog.setTitle(MY Title);
            pDialog.setMessage(MY Message);
            pDialog.setIcon(R.mipmap.ic_launcher);
            pDialog.setIndeterminate(false);
            pDialog.setMax(100);
            pDialog.setProgressStyle(ProgressDialog.STYLE_HORIZONTAL);
            pDialog.setCancelable(false);
            pDialog.show();

            return pDialog;
        default:

            return null;
    }
}
private void getthemetune(){
  mthemetuneref = FirebaseDatabase.getInstance();
    DatabaseReference ref = mthemetuneref.getReference().child(MYREF);
    ref.child("airwolf").addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            tvThemeTune = dataSnapshot.getValue(String.class);
            Log.i("Url At Capture",tvThemeTune);
            plysound();

        }

        @Override
        public void onCancelled(DatabaseError databaseError) {

        }
    });
}
private void plysound(){
mp = new MediaPlayer();
String j =
tvThemeTune.toString();
Log.i("Url",j);
try {
    mp.setDataSource(j);
} catch (IOException e) {
    e.printStackTrace();
}
try {
    mp.prepare();
} catch (IOException e) {
    e.printStackTrace();
}
mp.start();
Log.i("Sound playing", "Ok");
mp.setLooping(true);

}