There are a number of recordings of which the original music manuscript, printed score and parts are now lost. Edward Isaac's piano concerto and Geirr Tveitt's piano concerto No.3 are examples. I am wondering if there is any one out there who would be interested in transcribing them back onto paper.
Can you imagine just how difficult and time consuming that would be? And nearly impossible to do correctly? Well, it has been done. Pianist Alain Lefevre was given a recording of the 4th piano concerto of composer Andre Mathieu. The score/parts were lost and composer Gilles Bellamare transcribed it. And a beautiful work it is. You can hear it in a recording with the Tucson Symphony and get the whole astonishing story. Contact Bellamare. He seems to have what it takes.
And whilst he's at it, he can knock up a score for Bowen's 3rd Symphony (if, as suspected, the trail to the manuscript goes cold)! ;)
It is indeed very time-consuming and one needs to have a really first class ear, but it can be done. Perhaps not wholly accurately, but pretty closely. There are a number of works I personally would like to see reconstructed in that way - the PC by Lesley Bridgewater for one. However, it would cost a lot of money because fellas who've got what it takes will not charge peanuts (and why should they?) unless it's a work that is particularly dear to them.
one piece I am almost sure exists only on recording is a brief choral work (based on Shakespeare, I think...) by Benjamin Frankel - haven't heard it, heard about it from a family member. Recorded for the BBC, was broadcast, score either went pretty hopelessly missing(?) or is pretty certainly destroyed...
(edit 13 April 2012 - briefly: score found, in this case- my information was as of 1999 :) )
This is done a lot for film music, though I believe they have the composer written short-scores (that is what the composer gives to the orchestrator), at least.
in those cases where the orchestrator was someone else, anyway.