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.
External devices or third-party helper software may be to blame for startup/shutdown issues.
iMac Core Duo (Early 2006) Special Report: Problems with iSub -- potential cause and fix
iMac Core Duo (Early 2006) (#3): Safe sleep available; Problems with iSub
Keyboard becomes unresponsive on various Apple portables (#2): Fixes
Troubleshooting GarageBand: iSub distortion
More problems caused by System Migration from PowerPC to Intel-based Macs
Special Report: Troubleshooting the iMac Core Duo (Early 2006)
iMac Core Duo (Early 2006) Special Report: Keyboard volume/eject keys fail
iMac Core Duo (Early 2006) Special Report: Problems with flash memory devices