Ok, just to (temporarily I hope) close this thread:
Having Parallels and Windows 7 on my Mac, I have just installed YAMB and extracted the AAC streams of my three MP4 files. So easy!
In the process, I have also discovered "My MP4Box GUI", which is another Windows only application doing the same things as YAMB. Some people say that it works sometimes better than YAMB.
http://my-mp4box-gui.zymichost.com/
I have tried both on Parallels + Windows 7 and they gave me the same results.
But basically, both YAMB and My MP4Box GUI are just graphical user interfaces for MP4Box which is a command line tool MP4 multiplexer.
Conclusion: A little disappointment to have that much trouble for such an astonishingly simple task! Moreover, Apple is one of the main supporters of the AAC and MP4 formats...
It sadly reminds me why I was reluctant to switch to Mac.
At the same time, it is too easy to throw the stone; Windows still has around 90% of the computer market shares.
MP4Box being an open source project, if I have time I may take a look at the source code and create a simple Mac tool just for the AAC stream extract function.