I Still haven't found the exact solution to mute the Youtube Video using the Api.
Instead, Here's the work-around solution I've reached, in my case. Hope it might be helpful for someone else.
I've set the TTS Stream to STREAM_ALARM.(refer this question)
HashMap<String, String> params = new HashMap<>();
params.put(TextToSpeech.Engine.KEY_PARAM_STREAM, String.valueOf(AudioManager.STREAM_ALARM));
textToSpeech.speak("Hello World", TextToSpeech.QUEUE_FLUSH, params);
Now I Mute the SREAM_MUSIC Volume and I got the result I wanted. Anyone got better Ideas are more than welcome.
setVolumeControlStream(AudioManager.STREAM_MUSIC);
AudioManager am = (AudioManager)getSystemService(Context.AUDIO_SERVICE);
if (am != null) {
am.setStreamVolume(AudioManager.STREAM_MUSIC, 0, 0);
}