There are a number of ways to create fading behaviour in FMOD Studio:
Clicking and dragging the top-right or top-left corner of a sound module’s trigger region in towards the center of the trigger region creates a fade-in or fade-out curve for that particular module. Such curves are parameter locked, which is to say, the effect they have at any moment is dependant on the position of the parameter’s cursor.
An AHDSR modulator applied to the volume of an event track causes the volume of that track to exhibit fade-in behaviour when the event is started, and fade-out behaviour when it is stopped. (Note that to audition event stop behaviour, you need to hold down the stop button for 500 ms.) These can even be applied to master track volume to fade out the entire event.
An AHDSR modulator applied to any trigger region causes it to exhibit fade-in behaviour when the module starts, and fade-out behaviour when it stops. Note that AHDSR modulators are not parameter locked.
Automation can be used to create a variety of fading behaviours.
It’s not clear from your description which form of fading behaviour you need; If none of the above fit your purposes, please contact firstname.lastname@example.org with a detailed description of your needs, and we may be able to suggest a solution.