This is the circuit that generates the sound played when the ball hits the top or bottom of the screen. It uses the vertical ball counter.

When VVID is active at the same time that VBLANK is active, that means the ball has entered the vertical blanking interval, which means the ball has hit the top or bottom of the screen. This turns on the JK flip flop. While it's on, VBALL32 is used to generate the sound to play. VBALL32 has a frequency of about 490 Hz.

If SERVE is low, then the sound won't play. This is important because when the serve timer is running, the vertical ball counter is still active, which means the ball could still bounce off the top or bottom of the screen, even though it's invisible. So the bounce sound needs to be inhibited.

Wait for the Audio Out to fill up, then click "Play Audio".