Three excerpts. Which voice entered?
In each one, the voices start at different moments. Say which one came in last. Every voice uses the same instrument and sits dead centre in the stereo field — no timbre hints, no left-and-right. Only the line itself tells you where it is.
Headphones or decent speakers help. Takes about a minute.
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Which voice entered last?
Now you can see where each one came in. Hear it alone, then in the texture again — that is the motion the app trains: finding the line without stripping it out.
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That is still the easy version — you are only asked to name the voice. The app asks harder questions: sing the voice while the others keep playing, and follow it once it stops being the one that just moved.