I'm trying to make a sound play once, and then stop and not play again. Is this possible?

What we use to only allow something to run once, or only allow it to run once every x amount of seconds, is known as "debounce". A debounce can be any if statement that the script has to run before executing the rest of it. For example, if you have a variable in the part, you can do this.

 script.Parent.Touched:Connect(function(hit)
    if hit.Parent:FindFirstChild("Humanoid") and script.Parent.HasPlayed.Value == false then
        -- play the sound
        script.Parent.HasPlayed.Value = true 
        --[[since this value is true, it will never make it through the if statement again           
            unless you change it back to false.]]
    end
end)

You can also use debounce as a variable, however in your case the BoolValue would be much more effective if this is a single player game. The variable would look like this:

local debounce = false

script.Parent.Touched:Connect(function(hit)
    if hit.Parent:FindFirstChild("Humanoid") and debounce == false then
        --play sound
        debounce = true
    end
end)

Hope this helped.

I usually do this for example:

script.Parent.Sound:Play() -- it only plays once

You would have to set this to loop to play over and over. It's actually very simple! All it does is play it and then do nothing.