Have you looked at the “Music/Level 02” event in the “examples.fspro” project that’s included in every installation of FMOD Studio? It does exactly what you describe. The only change you’d need to make would be to replace the “Area” user parameter with a distance built-in parameter.
It sounds like you might be using a spatializer effect in your event’s master track signal chain to attenuate your music over distance. If so, delete this spatializer effect; if you’re using the method from “Music/Level 02” to automate individual track volumes, the attenuation from the spatializer effect is redundant and unnecessary. As a side effect, deleting the spatializer effect will mean your music is no longer panned to sound like it’s coming from a particular direction, meaning that it will retain the stereo width of the source audio files.