How to install USB-A driver for Harman Kardon Soundsticks on macOS 10.5.8 and iMac G4?

I don't know if this article might be helpful.

https://www.cnet.com/tech/computing/possible-fix-for-harman-kardon-isub-problems-with-powerpc-macs/

I am not familiar with the Open Source "iSub Fix" project that it mentions, and cannot recommend for or against it. The CNET article itself says "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."

It does appear that the "iSub Fix" files still exist (on SourceForge), although nobody has updated them in over ten years.

Any relevant drivers would be from Harmon Kardon and based on the age of that SoundSticks unit, they probably no longer offer them.

Apparently there were several versions of SoundSticks and at least some were developed in partnership with Apple. The big subwoofer unit is called an "iSub".

https://en.wikipedia.org/wiki/SoundSticks

"Apple introduced a software implementation to adjust the frequency range of the iMac's internal speakers so they would not overlap with the iSub, which increased the iMac's maximum volume as the speakers no longer had to produce deep bass."

The Wikipedia article shows the stereo speakers as being actual sticks (with multiple small cones per stick) - while your picture shows stereo speakers that look like the ones that came with the iMac G4/800.

https://support.harmankardon.com/on/demandware.static/-/Sites-masterCatalog_Harman/default/dw8b4dd2d5/pdfs/ISUB%5EHK%5Eom%5E%5Eml%5E%5E.pdf ("iSub Easy Setup Guide") says that you may download iSub driver software from Apple's Web site. But the setup guide is copyright 2000, and the availability of drivers may have changed in the ~25 years since it was written.