Real media video byte?
I have no idea what that stands for, but if it is that crapy real media stuff google mplayer, and get the codec packs, they work for xine.
install them to a path you can remember people seem to be fond of /usr/lib/win32 around here.
and edit/check the xine config file and make sure the path is listed correctly.
should be in /home/%user%/.xine/config
the section you need to check reads
# path to RealPlayer codecs
# string, default: unknown
# path to Win32 codecs
# string, default: /usr/lib/win32
That should do it I think.