I just gained the achievement, 'Short Reign' (Defeat the Skeleton King in 20 seconds or less), and then the sound became distorted as the game cycled through a number of different in-game sound effects.

-- I was in Normal mode
-- Was using a level 39 Monk brought in from Nightmare for the sole purpose of the achievement
-- Mantra of Conviction with Overawe were being used
-- The achievement was gained

The sound distorts upon the king's death and lasts while the loot drops, the king's death animation plays, and the UI's notifications of the achievement play. So something like 15-20 seconds.