Not sure about a freeware sound editor I know there is a payware one. Try fspanelstudio.com they have a payware editor. If you are trying to get FS to use new sounds that you have either made or imported from elsewhere I think you will have to make a sound cfg file for those files. But I wont swear to that as I think that the sounds in the effects folder do not have one such as thunder. But I could be wrong I have not really looked that good.