Depends what kind of flashcards I'm doing, & what I'm doing.

If I'm writing flashcards, I can listen to anything.

If I'm reviewing vocabulary flashcards, I need something without lyrics. Doesn't matter what genre.

If I'm reviewing flashcards with text/paragraphs, I try to listen to something without lyrics.

If I'm doing math flashcards or something that isn't necessarily read like a sentence, like logic problems, then I can listen to anything.

If it's boring, then I try to listen to something more upbeat to keep me on task.