Nice, but why doesn't it already have the sound Output preferences like that already? Where either the Minitoslink/Headphone output or iMac default speakers can be enabled?
BTW if I go this route with the adapter it doesn't effect sound quality since it isn't the native port?
For some reason when they designed the iMac, if you plug something into the built in audio out/headphone jack, it disables the built-in speakers. I believe that this is the way iMac is hard-wired and it is not a software/OS function (Though I could be wrong).
My prefs with empty headphone jack
with something plugged in headphone jack
The only way I found around this is to add a usb sound out adapter.
They sold are in various levels of quality from simple stereo to full 3d surround.
How they stack up against the built-in head phone jack in terms of sound quality, I can't really say. I'm not much of any audiophile.
But it does work and its how i connect to my reasonably mediocre home sound system.