AudioManager 를 찾아보세요.
http://developer.android.com/reference/android/media/AudioManager.html
사용법은 대략 다음과 같습니다.
AudioManager am = (AudioManager)getSystemService(Context.AUDIO_SERVICE);
am.setStreamVolume(AudioManager.STREAM_MUSIC, volume, AudioManager.FLAG_PLAY_SOUND);
* volume 범위는 0 ~ am.getStreamMaxVolume(AudioManager.STREAM_MUSIC)