I could tell the difference between them if you play them back to back. But if I woke up in the morning and listened to one of them, I wouldn't be able to tell

I could tell the difference between them if you play them back to back. But if I woke up in the morning and listened to one of them, I wouldn't be able to tell you whether it was the one in C or the one in Db. Not sure if there are recordings like this, but you could use GarageBand, Audacity, or the like to pitch-shift a recording without changing the tempo.