Play audio without having to watch the video:
Option 1 (Play audio from blank screen):
Load YouTube video & pause it right after it starts to play.
Press the power button once to turn the screen off.
Double press the home button and press play.
Option 2 (Play audio and use other apps):
Load YouTube video & pause it right after it starts to play.
Double press the home button, slide to the left and press play.