You could make interactive audio books with this! It would make the design a bit more complicated, but perhaps the card sliding motion could be used to read barcodes with 1 IR or power indicator LED and 1 or 2 sensors, for 2^8 or even more cards IDs.
I can see these devices being used beyond second language acquisition. Any type of memorizing declarative knowledge (facts, figures, name, specialized vocabulary) in a range of subjects: history, science, even some math concepts.
