Am I correct in thinking that SuperpoweredDecoder doesn't work with files straight from /assets or /res/raw? Do the files need to be copied somewhere else so that an absolute path can be provided?

Assets stored under /assets are zipped inside the APK file, so they need to be unzipped first. Storing compressed music under /assets doesn't make sense, as zipping them is not really useful. Assets stored under /res/raw are concatenated to the APK file. They can be directly accessed