That's what is in "Applications > Utilities > Audio Midi Setup" (sorry, the language, but "entrada integrada" = built-in mic; "fonte" = source; "entrada de linha" = line-in; "principal" = main; "mudo" = mute; "saída integrada" = built-in line-out):
At garage band preferences it says: "Estado do MIDI: 0 entrada(s) MIDI detectada(s)", id est, MIDI status: 0 line-in MIDI detected.
BDAqua. Thank you very much for your concern about my problem, but I just solved it! Or better, an Apple technician helped me. It was just a misunderstanding. There is no built-in mic in a mac mini, but an audio-in ("entrada de áudio") and and audio-out ("saída de áudio").
One cannot insert a simple microphone on the audio-in, but a mic with four stripes, like nokia's cellphones:
Once you connect such a phone on the audio-out hole, you'll be able to both listen and speak throught the headset.
After that, my "Applications> Utilities > Audio midi setup" showed a new section called "microfone integrado", i.e. integrated mic:
Hence, I was able to move "principal" to the right.
Besides that, I could not tet skype capturing my voice... But I think it's subject for a new topic.
Thank you very much for your help!