I've searched around on the forum, and audio not working on iOS devices (such as ipad, iPod etc) seems to be a common problem, as you can read about here: http://www.scirra.com/forum/audio-still-not-working-in-r119-on-ios_topic63619.html
I followed @davida13 suggestion to remove all m4a sounds from the offline appcache (to be found in the export folder, under the file name 'offline') as my game was suffering exactly the same problem, and it now works fine on both safari and Chrome. As Ashley mentions, the downside of doing this is the game won't work offline (except on desktop devices, which use .ogg format sound, I believe) so you may need to decide what's most important to your game - sound working on iOS devices, or offline support for devices that use m4a sounds.
Currently I am only running my game from Dropbox, but I'll be uploading it to various game sites shortly. I'll let you know if the offline audio solution causes problems, or makes the game work better later (unfortunately I have no Android devices to test on, so I cannot check whether fixing things on one device causes problems on another)