Search the net for music greeting cards and all you find are sites offering those annoying e-greetings that should've died out around 1998. The real paper cards that play music when you open them may be a cute childhood memory, but they can make an innovative ad tool, and the music doesn't have to be "Jingle Bells". You can record pretty much of anything up to 20 secs long, including your voice or your client's brand tune.
Here's a Google Answer on how musical cards work. Electronic123 sells DYI electronics kits. Chung Yen Products and Smartocean sell pre-made and custom-made cards (they also sell other cool stuff like flashing pins). Chrys's Corner sells regular musical cards with pre-recorded tunes (plus musical windmill and musical water globe).