If your license permits source code access for our UE4 integration, your audio programmer should be able to solve this problem simply by editing
FMODUtils::DistanceToUEScale to use a different ratio of unit conversion.
If you do not have a license that permits source code access, the only option is to double all the distances used in FMOD Studio. The easiest way to do this would be to use an FMOD Studio script that edits all your project’s spatializer effects and resizes all its distance built-in parameters. Information about FMOD Studio scripts can be found in the Scripting and Scripting API Reference chapters of the FMOD Studio User Manual.
It would be useful if there was an easier way to solve scaling issues like this when using UE4, so I’ve added a task to our feature/improvement tracker to make distance unit scaling a user-editable property.
Incidentally, it’s not entirely true that FMOD uses metres by default. Rather, FMOD has no awareness of what units your game uses to measure distance: When your game engine tells FMOD that the distance between your listener and an emitter has increased by two, FMOD does not know or care whether that means two inches, two kilometres, or two astronomical units; all are valid options, and FMOD treats all of them the same way. We encourage users to use metres as the default because a number of popular commercial game engines use metres, but it’s far from the only option.