I like the song "This is my country" from the RTS Democracy map. But when I open the .vpp in the builder I can't find the .wav file.

How is it hidden?

It's not inside the RTS Democracy vpp, you are right about that.

It's an ambient sound, and it doesn't play for me, which leads me to believe that you have another .vpp between your multi and single folders that has a .wav file named Music-RtsDemocracy.wav.

You're right. I found it. I wonder why that was done. Maybe to make it easier to remove the music if a user didn't like it.

I see a few maps do that.

While working on a map it's easier to put the music (and sound) files in a separate .vpp file. This is because every time you pack the level file in RED Editor you have to manually add the music files with the .vpp builder to the new file. After accomplish a map, i prefer to pack the music files into the final .vpp and not keep the seperate.