This can be set up entirely within FMOD Studio, without any additional code in Unity for calculating distances or adjusting volume.
You’ll want to add an FMOD Spatializer to your event’s master track, as you’ve likely already done, then set the “Envelopment” setting to “User” and increase the “Minimum Extent” to its maximum setting of 360 degrees.
Essentially, this makes sure that your event’s audio stays 2D, but is still attenuated based on the distance from your listener. You can tweak the minimum and maximum distances for the attenuation by adjusting the Min & Max distance event macro, which is located in the bottom right hand corner of the event editor when the event’s master track is selected:
You already have a listener on your camera, so if you play this event using an event emitter component, the event will automatically be “attached” to the component’s GameObject, and the 2D attenuation based on distance from the listener should be handled automatically.
If I’ve misunderstood the audio behavior you’re going for, or if you run into an issue getting it working, feel free to let me know.