I think most of the users know the answer but maybe it is too trivial to answer, the basic step how to use GarageBand (which is described in the User Guide)
You are getting caught up in words without understanding of the basic concepts. "Voice" just means that this is a template to record an audio signal (and not a MIDI signal). The setting is optimized with additional effects that might be useful when recording a voice instead of a guitar. But it is still an audio signal.
the important thing you have to "tell" GarageBand where the signal is coming from. If your audio signal (from the Cassette) is going into the Beringer, then you have to select the the Beringer as the Input Device in GarageBand so that signal can be recorded in GarageBand. That important setting is done on the firt window.
Her are the steps
This is the first window you see when you open GarageBand, or selec the New command in the File Menu
The Audio Input popup menu looks different depending on your setup. CHoose your Audio Device (Behringer)
Click CHoose and GarageBand creates the Project, and opens it
Close the Library Window on the left (you don't need it) and delete all the Tracks except the first one.
Now just hit the record BUtton and start your Cassette to record it
The controls on the bottom (Smart Controls) are optional to improve the recorded audio signal (Equalizer, COmpressor, Reverb, etc)
It won't get more simple than that, and the more you read the manual and the more you understand it, the simpler it gets.
Hope that helps
Edgar Rothermich - LogicProGEM.com
(Author of the "Graphically Enhanced Manuals")
http://DingDingMusic.com/Manuals/
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