I have the Teensy 4.0 with the Audio Board for 4.0
Trying to compile the Code Example:
Example/Examples for Teensy 4.0/Audio/HardwareTesting/PassThroughUSB

On a fresh installation of Arduino 1.8.10 / Teensyduino 1.49 on Windows 10
Documents/Arduino/libraries folder is empty

Arduino throws the following error:

Code:
PassThroughUSB:7: error: 'AudioInputUSB' does not name a type
To make an USB Audio device, use the Tools > USB Type menu
 AudioInputUSB            usb1;           //xy=200,69  (must set Tools > USB Type to Audio)

 ^

PassThroughUSB:9: error: 'usb1' was not declared in this scope
 AudioConnection          patchCord1(usb1, 0, i2s1, 0);

                                     ^

PassThroughUSB:10: error: 'usb1' was not declared in this scope
 AudioConnection          patchCord2(usb1, 1, i2s1, 1);

                                     ^

PassThroughUSB: In function 'void loop()':
PassThroughUSB:21: error: 'usb1' was not declared in this scope
   float vol = usb1.volume();

               ^

Multiple libraries were found for "Audio.h"
 Used: C:\Program
Multiple libraries were found for "SPI.h"
 Used: C:\Program
Multiple libraries were found for "SD.h"
 Used: C:\Program
 Not used: C:\Program
Multiple libraries were found for "SerialFlash.h"
 Used: C:\Program
Multiple libraries were found for "Wire.h"
 Used: C:\Program
'AudioInputUSB' does not name a type
Please note the odd and incomplete statement: "Used: C:\Program" ...
Using the Windows Explorer search on C:\ searching for "serialflash.h" only yields a result in
C:\Program Files (x86)\Arduino\hardware\teensy\avr\libraries\Serial Flash