The specifically labeled 'audio' ones were more expensive because the RIAA had a hand in them. The audio ones were sold with an extra fee built into the price because it was assumed you were going to burn pirated music to them....That doesn't make a whole lot of sense though, as they're much more expensive than damned old common CD-Rs.
All of that notwithstanding, my major point was, "Sony Music CDs", weren't simply a marketing ploy, attaching a different name to a standard CD-R. They had a specific application..