The other 8 GB is reserved for other stuff like apps. If that's the case I'm not sure what the free 8 GB internal is for because music goes into the micro SD card anyways.
16G internal storage... part of which is space the operating system (or what we usually call ROM) needs. Every Android device does this. The ROM needs space, your apps need space, your data needs space, the apps store a LOT of data, future ROM growth, temp space for downloading OTA updates, etc, etc, etc... 8G has been set aside for this need.