Is it possible by pressing a button in my Activity
to make the volume bar appear? The volume bar is the view that appears when you press one of the hardware volume
if you call setStreamVolume, pass the flag FLAG_SHOW_UI See http://developer.android.com/reference/android/media/AudioManager.html#FLAG_SHOW_UI
Working code
AudioManager audio = (AudioManager) getSystemService(Context.AUDIO_SERVICE);
audio.adjustStreamVolume(AudioManager.STREAM_MUSIC,
AudioManager.ADJUST_SAME, AudioManager.FLAG_SHOW_UI);