What’s the difference between 2D and 3D sound?
A 2D sound (or event) is played without any spatialization. It will sound the same as if you played it through a media player.
A 3D sound (or event) is played with spatialization. It will have volume attenuation based on the distance from the event instance and the listener, as well as panning based on the location of the event instance to the direction the listener is facing.
Just to add to my previous comment - an event doesn’t necessarily need a spatializer effect in order to be a 3D event. It can also use built-in parameters (such as distance, direction, etc.) to automate properties such as volume and equalization filters to also give the impression of spatialization.
You can use EventDescription::is3D to check if FMOD considers an event 3D.