My game uses FMOD as it's audio engine, and enabling both the Unity Audio System and the FMOD system at the same time will reduce performance. So it would be a nice feature to add FMOD support!
I don't use FMOD myself, but I'm looking up a few tutorials on in, and I think it should be doable? I'm not 100% about the implementation of it, but I think if I were to add a delegate event that got invoked every time the audio source is supposed to play and you hooked that up to a script that interfaces with FMOD, that might work?
You could also modify the source code and cut out the audio source entirely and replace it with FMOD I think, but I wouldn't be able to debug something like that!