The Chinese like to share their movies in RMVB (RealMedia) format as opposed to Xvid or DivX. Not sure why it got to be so popular (I would prefer the more open Xvid), but the file sizes and quality it produces are quite good.
A problem I ran into is that mplayer/vlc don’t seem to play these files without a bit of tinkering. I found a lot of how-to’s that tell you how to make it work, but non really straight forward on a 64-bit system.
Mplayer supports a number of codec libraries though, and more than one may get you result.
Where many how-to’s told me to install the RV40 codec (which I couldn’t find in 64-bit), I used the RV3040 that comes with Ubuntu by default instead. This fixes video.
For sound, the often mentioned solution was to install the w32codecs and use the cook codec. In my case however this resulted in choppy sound. Instead I use the ffcook codec (found in libmad) instead (also in Ubuntu by default.)
Finally my command line looks like:
mplayer -vc rv3040 -ac ffcook movie.rmvb