Cool birthday present, I'm sure your girls will love it :)
It's probably best to use a variable for triggering sound fx. Just set up your sound trigger event, so that it only fires when your variable = 5. Then add 1 to the variable on every coin pick up. Don't forget to set the variable back to zero once the sound has been triggered, then it should start over again.
Good luck, and I hope your daughters have a wonderful birthday!
Edit: just saw your second post :( I was using Chrome and it all seemed fine there!