Generate a sound (not from a file) in Swift / SpriteKit

I'm building a small game prototype, and I'd like to be able to play simple sounds whose length/tone/pitch will vary based on what the user is doing.

This is surprisingly hard to do. Closest resource I found was:

http://ift.tt/1zHlqoU

But this does not actually generate any sound on my device or on the iOS simulator.

Does anyone know of any working code to play ANY procedurally generated audio? Simple Sine Wave would do.