Hello Ashley, I concur with blacksmith, it is know that the audio is different between devices.
I have also tried building and exporting the attached capx and there is no sound on my iphone 5
I appreciate that there is an expense associated with buying a device for testing, however the iPad air 2 makes up 3.5% of the apple device range while the iphone 5&6 cover more then 50% (http://hwstats.unity3d.com/mobile/device-ios.html). Can I suggest getting a second hand 5 to improve coverage (£100ish).
Is the iPad Air 2 the only device that you are able to test iOS builds on? Is there anything like logs or such that I could send you to help testing on iPhone 5.
Regards Philip