Is the data stored on the card, or on the player? My guess is that each card just holds an id?
And second the blank/customizable cards, that's what 80% of our cards are and my daughter loves helping track down and extract content. Biggest hits for her have been Roald Dahl and random science stuff.
Also David Tennant reading the How To Train Your Dragon series. He is a superb reader with an amazing range. Before he hit the big time with Dr Who he did a lot of radio.
We've also had family members record stories and I've put them onto cards. We gave my son a dictaphone to record his own stuff. He would do great long sagas of mayhem and battle, but he's lost interest in recent months.