I had installed Rune Gold, but couldn't help but notice how oddly quiet the game was. I looked in the Rune Gold -> Music folder and noticed there was nothing in there. Is this normal?

The music folder is normally unused. Music from the single player is instead stored within the sounds directory.

Rune classic however should have an .ogg format version of the Rune OST in the Music folder. (And I believe this would include the tracks from the original Halls of Valhalla CD as well?)