It's strange cause I was writing similar functionality and Android seems to play louder sounds when you rise stream volume.
am.setStreamVolume(AudioManager.STREAM_MUSIC, progress,AudioManager.FLAG_PLAY_SOUND);
Thats what I used in my application. am is instance of AudioManager you can get by writing:
AudioManager am = (AlarmManager) getSystemService(AUDIO_SERVICE);
To disable sound you can replace AudioManager.FLAG_PLAY_SOUND with "0" value what should disable it.
I'm not sure if it is possible to replace those sound in AudioManager but you can play those custom sounds with MediaPlayer inside of your onKeyDown methods.
Hope this helps.