Xperia S Backup & Restore: Could not back up media files and downloaded apps with data
You do not need to back up media files or app data. They are in device's internal storage. If  you are performing a factory reset these datas wont be erased unless you select Erase Internal Storage when performing a factory reset.

If you still need to backup the media files or app data, connect your Xperia S to a PC and backup the files in the PC.