If your iPad is running iOS version 6.1.2 or later, you may find that audio doesn't work when you resume HTML5 content in mobile Safari. (Click here to determine which version of iOS you have.)
This issue was corrected in Update 3 for Storyline. To take advantage of the new features and fixes, download and install the latest version of Storyline from the link in your product confirmation email, or get it here. Then republish your Storyline project.