How Add Piezo Buzz to Pong Sketch
The condition HIT_CENTER || HIT_RIGHT || HIT_LEFT will always be true.  (All three of those HIT_ values are non-zero constants.)

Instead, you need to test if getHit() is non-zero.  (To see this, search the sketch for occurrences of HIT_RIGHT or HIT_LEFT.)  

The sketch sets up an event timer (via ball_timer = timer.every(BALL_DELAY, moveBall); in setup()) so that ball position will update every 200 ms.  The loop() sequence is: Use timer.update() to service timer events.  Use setPad() to read the paddle position.  Use drawGame() to update the game picture. Somehow use checkLoose() to see if game is over.  Use delay(GAME_DELAY) to wait 10 ms.

Given that structure of loop(), throwing in a 100 ms delay to wait for a buzzer would mess up the game play.  You could turn on the buzzer and set up a timer event for turning it off, each time the ball hits the paddle.

More straightforwardly, since a 10 ms beep is likely to be adequately long, you could test for a hit just before delay(GAME_DELAY), and always turn the buzzer off just after that delay.  In other words, before the delay say:
if (getHit()) digitalWrite(buz, HIGH); and after the delay say digitalWrite(buz, LOW);.

This supposes that writing HIGH to buz turns on the buzzer and that writing LOW turns it off.