Thanks Daniela!
My car slider has 50 steps, so I added two triggers for the audio to play: play when the car slider moves and is equal to 1 (so as soon as car starts moving) and stop playing when the car slider is equal to 50 (so at the end). Then, to loop the audio I added another trigger to play the car sound when the car sound completes, on the condition that slider car is between 1 and 49.
So that worked fine, BUT, when I dragged the car back to the beginning, the sound would play again, even if I just left the car at the start without moving it. So, to ensure that the sound only plays when the car is being moved, I used a variable (which I named audio, I think) and added a trigger to toggle the variable audio when the car slider moved if it's equal to 1, and put it at the top of the triggers list. Finally, I added a condition to the first trigger (the play sound trigger) so that it would play sound when slider car moves IF slider car is equal to 1 AND the audio variable is equal to TRUE.
So this happens: car moves to 1, audio variable toggles to TRUE, car sound plays (because audio var is TRUE)... when you slide the car back and it goes past '1' the audio variable will toggle back to FALSE so the play sound trigger will not fire. If the user tries again, the audio var will toggle again to TRUE so the car sound plays again.
Hope this makes sense :) If it helps to see this, you can grab the SL file here:
https://drive.google.com/open?id=0BysKZkRKOzlfNUltN3lyMEF6cVk