I just clip the holster of my Blackberry Tour to a wire tie on the stoker's handlebars, and wallah! Satellite radio via the Sprint Network and Pandora. The speaker is loud and clear enough for both of us to hear, and the stoker can manage the volume, change channels, etc as she likes. The music adds a lot to our rides, plus we have the phone and GPS capability if needed.
I just love telling people who ask about our tandem that it has Michelen tires, satellite radio, and color TV!