Quizlet is probably the most well-known digital flashcard tool. Quizlet lets students create flashcards from scratch, import lists to create flashcards, or browse for sets of flashcards created and shared by others. Flashcards created from scratch can include pictures and audio (audio is available only with a paid upgrade). The flashcards students create can be viewed online and on their mobile devices even if they lose their data connections.
If you want to create flashcards that include audio for free, Flippity's flashcard creator is the way to go. Through Flippity.net you can quickly create a set of flashcards that include pictures, videos, and text. You can even turn your flashcards into a printable quiz. In the video embedded below I demonstrate how easy it is to use Flippity to create flashcards from a spreadsheet.