@Override
    public void applyEffectTick(LivingEntity livingEntity, int amplifier) {
        if(!livingEntity.level.isClientSide() && livingEntity instanceof Player player) {
            Double x = livingEntity.getX();
            Double y = livingEntity.getY();
            Double z = livingEntity.getZ();

            livingEntity.level.playSound(player, x, y, z, KCSound.WHAT.get(),
                    SoundSource.MASTER, 1, 1);
            livingEntity.teleportTo(x, y, z);
            livingEntity.setDeltaMovement(0, 0, 0);
        }
        super.applyEffectTick(livingEntity, amplifier);
    }

I have an effect code that needs to freeze and play a sound. The problem is that the freeze works, but the sound does not. How can this problem be solved?

livingEntity.level.playSound(null, x, y, z, KCSound.WHAT.get(), SoundSource.MASTER, 1, 1);

If you pass a player as the first parameter then that player is excluded from the network broadcast.

Passing the player is intended for when have some other mechanism to play it for the player that caused the sound. e.g. you already played it on the client before it even got to the server.