I was looking through my burned CD-Rs, and found a copy of Winamp 5.1 that I've downloaded back when it was released. With a little extracting using 7-Zip on the installer, I've now found the "demo.aac" file which is what you are looking for. And yes, this file was created exclusively for 5.1 surround speakers, and version 5.1 of Winamp is the only version carrying that file. Later versions use the generic demo.mp3. So there you have it!
I've attached the ZIP with the demo.aac file for download to anyone who has a 5.1 surround sound system and/or a sound card that supports 5.1.