I was browsing all 3 electronics candy stores (Adafruit, Sparkfun, and Digikey). I noticed that the Teensy 4.0 version of the Audio Shield hasn't made it to Adafruit or Sparkfun yet (Digikey gets their supply from both). I was actually looking to buy something from Digikey and combine shipping, but instead I went with PJRC.com directly.

It looks like Adafruit is getting prepared for the new audio shield, in that they've changed the title to be "Audio Adapter Board for 3.0 - 3.2, 3.5, and 3.6" while Sparkfun still has the "Teensy Audio Adapter Board" as the title. I hope they get the stock soon, and that Sparkfun changes their title to avoid confusion.