If you generate the project (F7), it will create the source files necessary to compile and link the project, including the binary data for the sound. Find the folder the .SGDK2 file is in, look for the folder underneath that by the same name as the file, and find the file with the same name as the sound object in the .SGDK2 file, with a .bin extension. If your object is called Music.cs, find Music.bin. Then copy it somewhere else and change the extension on the file to the original file type, and there's the original music.