Sorry Guys, answered my own question.

Had to preload the audio for the iPad.... don't have to on other devices, or maybe it was just the browser.

Glad you figured it out, but just to add something in case other people have similar problems.

The best format to use for iOS Safari is .aac. However aac is just a container, there are a lot of different aac formats and not all of them work. The best way of making sure you have audio in the right format, is to convert it through iTunes. This will generate an .m4a file - you can simply rename that to .aac.

Also if you're targeting modern browsers you can click the WebAudio only option in the scene properties. By doing that, you can avoid preloading the audio. However this will stop audio working in Internet Explorer (though it will still work in Microsoft Edge).