Quicktime under Mac OS X Leopard can’t play movie or video files saved in .avi format encoded with DivX format without a little help. Also if you get no sound from avi files on your Mac, the following avi audio codec will solve that problem for you.
Here’s what you need to make Quicktime on Mac OS X Leopard (10.5) play .avi movie files:
Quicktime Video Codec – Free Xvid Quicktime Component for DivX codec avi files
Quicktime Audio Codec – A52 avi audio codec for Quicktime
(Thanks Orca for the updated A52 link)
Download both of these disk image files (.dmg) and double click them to mount these files (make them visible and accessible through Finder as another Device (top left hand corner of Finder) on your Mac.
Next we need to put these components into the right folders. Within Finder, click on the Xvid Alpha device. Within this Finder window, you should see a file named Xvid_Codec 1.0 alpha.component. You need to copy and paste this item into /Library/QuickTime/ folder on your Mac. The easiest way to find this folder is click on the first device (looks like a metal hard disk) within Finder (mine is called Leopard), then find the folder named Library, and within that, another folder named QuickTime. Paste the Xvid Alpha file into this folder (or drag and drop it if you have two Finder windows open).
For the audio part of playing .avi files in QuickTime you need to place the A52 codec component into /Library/Audio/Plug-ins/Components/ folder. Luckily the author of the A52 audio codec for playing avi files in Mac made shortcuts right within the .dmg file for A52 Codec. When you double click the A52Codec .dmg file you’ll see two huge arrows pointing from the codec files to the folders they should be dropped into so simply drag and drop the two files into the folders (which are actually shortcuts to the correct folders on your Mac machine).
If you want QuickTime to recognize and play Dolby AC3 encoded audio from movies, copy the AC3MovieImport component into the /Library/QuickTime folder as well. This definitely won’t hurt and you’ll probably enjoy it later.
After this if you already have QuickTime running, Quit QuickTime (Command + Q), don’t just Close it, since QuickTime will still be running in the background. You need to fully quit QuickTime and restart it for the codecs to be loaded by QuickTime, so this step is necessary.
After all this try double-clicking your .avi movie file again and see if both audio and video are now being displayed by QuickTime.