When Apple released some of the final updates to OS X 10.5 for PowerPC systems, a number of Harman Kardon iSub owners found their speaker systems no longer worked properly with their computers. The problem appeared to be a driver-related error but neither Harman Kardon nor Apple stepped up to fix the issue and owners had to resort to other speaker systems.
Recently, an open-source project "iSub Fix" has become available, which is a small driver replacement for PowerPC-based Macs running OS X 10.5.8, which should get the iSub working again. The package comes with a small installation script that essentially moves the three included driver extensions into their locations and adjusts permissions on them accordingly.
Download the extension package here: http://sourceforge.net/projects/isub-fix/
Keep in mind this project is an open-source and beta project, and it is likely the software has not been fully tested so use at your own risk; however, since the software is just a drag-and-drop replacement of three kernel extensions, it should be easy to undo.
Be sure to back up your system before installing this fix, which can be done via Time Machine or a clone. Alternatively, you can copy the following files from the /System/Library/Extensions/ folder to your Desktop or another safe location:
Once this is done, run the installation script according to the included ReadMe file, and you should be good to go after a restart.
If the new drivers do not work properly or give you other problems, you can revert the installation by moving the backed up kernel extensions to the same folder as the script and running it in the Terminal as you did to install the fix. The script should replace the backed up files in the same way that it installed the altered ones.