FMOD already takes care of this for you. You don’t have to develop your own system for it.
The FMOD Engine automatically determines when a channel (a track or bus) has become inaudible due to volume attenuation, and “virtualizes” it. Virtualized channels do not play or process their audio content, and so event instances with virtualized channels consume only the infinitesimal amount of resources required to update their timeline and parameter values. If a virtualized channel’s volume increases enough for it to theoretically become audible again, FMOD automatically makes it real (non-virtual) once more - with the result that the event instance resumes playing with its current timeline and parameter values, creating the illusion that the instance continued playing while out of earshot.
If you’re still concerned about resources, you can use FMOD’s built-in event instance stealing behaviors to customize how your game culls excess event instances by setting the max instances and stealing behavior properties of events and buses.
You can read more about instances limiting, stealing, and virtualization in the Stealing and Virtualization section of the Advanced Topics chapter of the FMOD Studio User Manual, and more about stealing in the