Such a device would be tremendously useful to musicians and composers, professionals and amateurs. Using it you would be able to score down your favourite tunes precisely, analyse and reproduce various natural sounds like waterfall, murmur or chirrup of birds.
It could be something compact like a score-sheet printer with a mike attached that would analyse sound pitch, duration, tone, rhythm etc. It could be used for rearranging some melodies for other musical instruments and for coverting human voice into pure instrumental melodies.