Well, I fixed something.
I was about to post that I think Apple changed iTunes some time back, removing the old AppleApplicationSupport, so that it did not include the required components for QAAC. Hence, on your old computer with an old version of iTunes, the components exist and are not removed. But a new installation of a recent version of iTunes, and specifically the Windows App Store version, will not install the components. At least this is what I remember. A history of iTunes may confirm that.
Anyway, I thought that I installed something, which registered the DLLs in Windows, and then QAAC worked. But maybe it was just my old iTunes installation that provided the CoreAudioToolbox.dll component. I have copies of CoreAudioToolbox.dll in multiple locations on my PC, and all have been placed there by an installation process. See the first attached image. While it sounds like QAAC looks in the current directory for the CoreAudioToolbox.dll file, and hence your command line test works, MC may require the DLL to be available to simply be called, which means it is in the Windows path or a registered DLL. This isn't clear to me. Maybe it should work by just placing the CoreAudioToolbox.dll component in the same directory as QAAC, and MC would use the Current Directory as defined in the External Encoder parameters to find it.
Read this:
https://hydrogenaud.io/index.php?PHPSESSID=rfubatgk0dl4vfuisidqrec185;topic=117089.msg971665#msg971665 It says that the CoreAudioToolbox.dll file should be in the same directory as QAAC files.
Plus this:
https://github.com/nu774/qaac/issues/59The version of QAAC I installed is qaac_2.64.zip. The current version is qaac_2.68.zip, which you can get from here:
https://sites.google.com/site/qaacpage/cabinet I assume that is what you are using.
The installation instructions are pretty simple, from here:
https://github.com/nu774/qaac/wiki/Installation I did acquire the LibFLAC components as I was converting FLAC to M4A.
My QAAC directory, which I believe I manually created, doesn't contain the CoreAudioToolbox.dll file. See the attached second image.
Just to round out this post, my QAAC parameters are shown in the third attached image.
TL;DR If you have CoreAudioToolbox.dll in the directory next to QAAC64.exe, it should run from within MC, if the External Encoder parameters point to the directory, as per my settings.