Vintage Breeze

“The song “Edelweiss” was written for the musical and is little known in Austria. The song was the last that Oscar Hammerstein II wrote before his passing in 1960.”- imdb.com

Also, it’s not Christopher Plummer’s voice that you hear, but Bill Lee’s. Christopher Plummer got quite angry they decided to replace his voice, which was as he later claimed, one of the reasons he accepted the part in the movie.