Were you trying to change permissions as root? If root owns it, only root can change permissions.
The command 'file /usr/lib/libavformat.so.50' will tell you what type of file this is. ( without the quotes ). If it is a link, run as root the command 'locate libavformat.so.50' without the quotes. You may get a message about the data base being more than 8 days old. Run the command as root 'updatedb' ( no quotes ). That will take a few minutes the first time. Then re-run the locate command. The system should list the path to all files called libavformat.so.50. Go to what ever path you find and adjust the permissions with chmod.