AVI torrent, with no sound...quicktime issue?

Install VLC Media Player and/or Perian. Repair permissions and restart your computer afterwards.

Repairing permissions is important, and should always be carried out both before and after any software installation or update.

Go to Disk Utility (this is in your Utilities Folder in your Application folder) and click on the icon of your hard disk (not the one with all the numbers).

In First Aid, click on Repair Permissions.

This only takes a minute or two in Tiger, but much longer in Leopard.

Background information here:

http://docs.info.apple.com/article.html?artnum=25751

and here:

http://docs.info.apple.com/article.html?artnum=302672

An article on troubleshooting Permissions can be found here:

http://support.apple.com/kb/HT2963

By the way, you can ignore any messages about SUID or ACL file permissions, as explained here:

http://support.apple.com/kb/TS1448?viewlocale=en_US

If you were having any serious problems with your Mac you might as well complete the exercise by repairing your hard disk as well. You cannot do this from the same start-up disk. Reboot from your install disk (holding down the C key). Once it opens, select your language, and then go to Disk Utility from the Utilities menu. Select your hard disk as before and click Repair.

Once that is complete reboot again from your usual start-up disk.

More useful reading here:

Resolve startup issues and perform disk maintenance with Disk Utility and fsck

http://support.apple.com/kb/TS1417?viewlocale=en_US

For a full description of how to resolve Disk, Permission and Cache Corruption, you should read this FAQ from the X Lab:

http://www.thexlab.com/faqs/repairprocess.html

Open the file using QuickTime Player.
Open the Movie Inspector window to find out what codecs are used in the file.

Check HD>Library>Quicktime