I've listened to Casey explaining the reason for fixing the frame rate on this game.
I've played many games where the frame rate varies depending on what is rendered at the time.
Plus these games have the option to disable VSync, so the frame rate varies wildly.
How do they calculate the sound output without the fixed framerate?

Most AAA games do not try to have tight sound sync on the PC. Usually the sound is just very laggy. Their game loops typically make no attempt to guess when the sound will play, they just stay ahead of the write cursor and leave it at that.

- Casey