Since mplayer is the only multimedia player on my PC other than Amarok, so I will obviously want it to work with each and every media file I have or may encounter and mplayer did all its work with 100% perfection till one day when I tried to play .rm/.rmvb files with it.. So what next, install a separate player for that? Never..the solution is more simple than anyone can expect :
1> If you have not installed the codecs for mplayer then download them from here or mirrors. Extract the contents and copy them to /usr/lib/codecs directory.
2> Now for enabling Real Player support, download this or mirror, extract the contents to the above mentioned folder.
Note: For copying files to /usr/lib/codecs u will need administrative rights (sudo)
TIP: For opening any file with mplayer (CLI), Right Click File > Properties > Open with > Add > xterm -e mplayer > Click on "Always open with same application". This is quite fast and handy.