Why not use getResource() instead of getResourceAsStream()? The former should return a URL instance.
Note that SoundSytem was original designed to load files from inside the JAR (the URL thing was added as a feature request later on). Every command that takes identifier + URL parameters has a version which simply takes a filename. By default, SoundSystem looks for the file in a package named "Sounds", but you can change it via SoundSystemConfig.setSoundFilesPackage(). If the files are under subfolders under a common root folder, call setSoundFilesPackage for the root folder, and then append the subfolder to the front of the filename when calling a method like loadSound. If there is no common root folder for the sounds, you should be able to pass an empty string to setSoundFilesPackage (haven't actually tried this latter option, but it should work)