Hi Khai,
Rachmaninov's Vocalise is a favourite of mine also although I don't have the sheet music and therefore haven't attempted to play it. For me one of the more interesting recorded versions uses a Theremin for the solo voice. If I remember correctly it was played by Lydia Kavina. There is also a Theremin performance available from Clara Rockmore.
Cheers
Len C.
[This message has been edited by manorcourt1991 (edited 01-06-2005).]