How do I limit audio to play just once on first visits to slide?

I was trying to the exact same thing as described above.  I had a variable set to "false" that would allow the audio to play on a layer the first time the menu slide was entered.  I would then set the variable to "true" which meant the audio wouldn't (shouldn't!) play when the menu slide was re-enterd.  I even displayed the variable on the menu slide and could see that it was true but the audio still played.  As mentioned by Bruce Graham, above, the following also needs to be done:

Hide the layer when the audio completes, (or you can use the gear to close layer when timeline ends)

It seems like it should be more clear cut and not play based solely on the condition of the variable but I guess that's something unique to Storyline.  At least now it works!