So, here is the "test story" on my Yikes G4.
Below you will find some screenshots made to illustrate my writing.
Sorry, my MacOS 9 is in french so the screenshots are too… :-/
Screenshot 1 : nothing is connected to the mac
Screenshot 2 : when I connect earphones on my mac, the last tab became available
Screenshot 3 : if I click on it very quickly after connecting the earphones, I can see during few milliseconds these pictures of speakers
Screenshot 4 : then the tab content switch itself to this
Screenshot 5 : then I connect the USB Audio adapter (does the same which the 2 I have bought) and the tab content switch to this
Screenshot 6 : here is the Apple System Profiler of this mac; you can check the USB drivers version => 1.5.6
In the output tab, I still have only one choice which is "Integrated" even with the USB adapter connected and working
after a very quick check, my mini is using the 1.5.9 …
I have to do more tests now on the mini by replacing drivers with the ones from my Yikes to see if it changes something