If you have latest RealPlayer and MPlayer installed, there's no need to remove them.
Do this:
- Install the rp9codec.
- Locate your installed RealPlayer, right click and browse into /Contents/Frameworks/HXClientKit.framework/HelixPlugins/
- Copy the "Codecs" folder to somewhere easily accessible.
- Rename "Codecs" to "codecs". Lowercase C.
- On finder, Go --> Go to Folder... or press "CMD + Shift + G" paste this "/usr/local/lib/".
- Copy the "codecs" fodler into the /lib folder, click Authenticate.
- Edit the plist of Mplayer as lhwbrian mentioned.

here's my feedback after few days of testing.

on some mac, the voice and video doesn't sync correctly, this can be fixed by changing the Video output module to CoreVideo or Quartz.

on some older mac (in my case G4), the performance is poor. any solution?