It's not exactly a doddle to 'convert' audio to MIDI. The app must analyse the audio material and extrapolate pitch, note length, dynamics etc (independently from its sound!) While it's perhaps RELATIVELY easy with a monophonic line and a simple waveform or very short 'atonal' events (like a drum beat), I can imagine that it requires some quite sophisticated algorithms to do that with chords or even more complex material.
Nuendo does not provide this in standard (yet) but there are a few third party plugs like mentioned Widi, Digital Ear etc. (I don't know any DAW software where this comes standard btw.)
If you've got the material as individual audio tracks, all the better, Widi will do the job quite ok. If you've got only a mix it'll be difficult...
l8rz
B#