Replacing QuickTime 2.0 which comes with System 7.5 and is preloaded on the Performa 6115CD with the QuickTime 1.6.1 (which comes on the Grolier Encyclopedia CD) is likely the issue. To correct this issue, update to QuickTime 2.1 or newer, which includes fixes for several issues encountered when working with multimedia titles. QuickTime software can be found at:
http://www.apple.com/quicktime/
This same issue could occur with any member of the Performa 611xCD family of computers.