If all you want to do is be able to access a file with a relative name to a certain directory, you can just use plain File API, like this:
final String baseDir = MainClass.class.getClassLoader().getResource("").getPath();
final String relativeName = "audio/abc.mp3";
final File relativeFile = new File(baseDir, relativeName);
final String canonicalPath = relativeFile.getCanonicalPath();
It should work on both Unix and Windows, since Windows accepts both Unix- and Windows- style paths (just saying it from my own experience).
