I think capability of Fmod bigger than in GS, for example GS sound system(DirectSound) do not support dynamic sound effects such echo,gargle,compressor and other, FMOD do not use windows stream so you can use big audio files without codecs, also with FMOD you can change position of playing file,record from microphone and use many function what exist in FMOD.
HeelX make very useful tool for all GS users.