They exist, but under generic implmentation. Again FPlatfrom classes are platfrom specific implmentations and only Generic one is in documentation, when you build FPlatfromMath is replaced with FPlatfrom Name HerePlatfromMath, here entire list of platform classes it had lot of useful stuff if you want to access platfrom OS and hardware information:
But yea i was trying to find Fmod but i could not found Fmod, something wrong with documentation search engine worth bug raporting