How do I make this audio event play?

Going to introduce you to anonymous functions.

Now, to keep them from playing the sound too many times, you can use a debounce.

Debounce makes it so they can't hit it more than once.

I'll also make it so if the players hits it, it will only play for THAT player, and not everyone in the server.

local hasHit = false --They haven't hit the part yet
local soundBrick = game.workspace.ScareBrick --Where the Sound's parent is located

soundBrick.Touched:connect(function(partThatHit) --This is an anonymous function
    if partThatHit.Parent:FindFirstChild("Humanoid") and game.Players[partThatHit.Parent.Name] and not hasHit then --If the model has a Humanoid, they are in the Player list, and they haven't hit it yet
        local plrCharacter = partThatHit.Parent
        local sound = soundBrick.Sound:Clone() --Getting the sound
        sound.Parent = game.Players[plrCharacter.Name].PlayerGui --Putting the sound in the player's PlayerGui
        sound:Play() --And play!
        hasHit = true --Now they hit it!
    end
end) --After the function, it requires and end with a ) after it