An audio tape is a type of tape used for audio storage and playback used in conjunction with early Federation holodeck technology.
When Uhura and Sulu thought they heard laughter in the recreation room, McCoy shrugged it off as one of the audio tapes rewinding. Unbeknown to them, it was actually the USS Enterprise's computer, which has acquired artificial intelligence, that was in fact laughing as it plotted its next practical joke. (TAS: "The Practical Joker")