it's pinned here in the downloads section. You can copy it to CD and then install from there, methinks.
The fact sonic stage won't recognize a file but WMP will play it means it's safe to "delete" from SS (don't "delete the music file from the computer") and reimport the oma file. DO NOT System Restore or reformat etc until you are quite sure that you have decrypted the files. Only way to test that... copy some and try to play on another machine that has SS.