I have now fixed my problem, although it took me a while to sort out. I removed all quicktime 4.1 extensions + the control panel and reinstalled 4.03. This still didn't fix things, until I removed the Sound Manager extension, and then ran the 4.03 installer again.
Before I fixed the problem, I tried double click individual error sounds held within my system file - this gave error -201, which corresponds to 'NotEnoughHardware' apparently.