FileMaker Pro 18.104.22.168)
MacOS High Sierra
The particular file I refer to is the one with the Luminous theme, in the group called "Sample", not the group called "starter" however it could exist in both of them, I have only been using the one with the Luminous theme. In this file, there is a field called File Container, which exists on the "Content Details" layout in each of the 4 groups of layouts: Desktop, iPad, iPhone and Web Browser. The problem is that in many instances of this field, the ratio button in the Inspector for "interactive content" has not been chosen. I have not taken the time to figure out which is which, because the layout is complex with layered slide controls and panels, but I can say with certainty that if you drag & drop an audio file into this container, the playback control only appears in the Desktop and Web Browser layouts. I can't think of a reason why it should not allow iOS users to playback an audio or video media file.
On the desktop layout, drop an MP3 file into the container. Then go to one of the iOS layouts and you will see it cannot be played.
Workaround: figure out which fields are the suspect ones and change them in the inspector. But be aware that the starter solution, unless fixed, will always have this defect.