When Apple launched its online iTunes music store not long ago, it promoted the new service with a set of distinctive ads. The spots (see them here) each featured an individual, against a white background, listening via headphones to an iPod portable music player and singing along to a favorite song. We couldn't hear the song, of course—just the person, singing a cappella, with or without musical ability. You could love these ads or be driven to distraction by them, but they stood out. More recently another music-download service has emerged, BuyMusic.com, with its own commercials. (See them here and...