Melodyne Direct Note Access (DNA) is what you are looking for to do this directly from a mixed-down file. http://www.celemony.com/cms/index.php?id=dna

I don't have any claims of using this myself - it's a rather expensive feature that I can't imagine works terrifically well, but their demos using a Chet Baker audio recording are almost scary in how well they can un-mix the baked cake, to borrow from Robh's great terminology above.

Now, that recording in the demo had at most 5 'voices'; Chet's vox, trumpet, piano, drums and string bass. I don't know what you would do trying to pull the 2nd violin part out of an orchestral recording. They manipulated the melody directly, without affecting any of the other instrumentation. It was very 'futuristic' and disconcerting at the same time. Just do a YouTube search for 'Celemony Chet Baker' and I'm sure you'll hit the demo videos.

When Celemony released that demo, it caused quite an uproar that it was like the Soviets retouching photographs and removing or adding persons from photographic history.

However, I'm of the opinion that all tools can be used or abused - I think this one could have real potential to help the songwriter play with a hummed melody or throw in a minor where there was a major, etc - with audio. All the things we are used to doing with MIDI and do it without guilt or shame; now potentially available with audio.

-Scott

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