You can not set it to record internal sound, but if you want to record audio that is coming from your computer there are options. The first option is to find a male to male mini, then plug one end in your head phone jack and the other in you ear phone jack. The other option is a program called audiohijack which will do exactly what you want, but not put the audio into garage band, however, you can simply import that file into garageband after the file is saved.