Get Dr. Divx from
here , you can use it for 15 days for free and it's official divx product and very easy to use.
also it will install divx codec 5.1 automatically, so you don't need install divx codec seperately and also it's better that you uninstall perivious divx codecs.
Reality player support both mp3 and AC3 sound and one advice, keep your frame size under 640x480 (ex. 512x384) for smooth playback.