Hi,
I tried to compile the example WavFilePlayer.ino 1:1 but it reports only errors:
I use Arduino 1.8.9 and I have copied the last version into the folder Arduino/libraries/
I tried the example from my local pc from example-folder and from the Github-Repo. Both did result in the same error:
Days ago I already played around with the Audio-Library and was successful to compile other examples which use Output-I2S too.
Perhaps Arduino did an update?

/Arduino/libraries/Audio/output_i2s.cpp: In static member function 'static void AudioOutputI2S::isr()':
/Arduino/libraries/Audio/output_i2s.cpp:152:58: error: 'arm_dcache_flush_delete' was not declared in this scope
arm_dcache_flush_delete(dest, sizeof(i2s_tx_buffer) / 2 );
^
Multiple libraries were found for "Audio.h"
Used: /Documents/Arduino/libraries/Audio
Not used: /Applications/Arduino.app/Contents/Java/hardware/teensy/avr/libraries/Audio
Multiple libraries were found for "SD.h"
Used: /Applications/Arduino.app/Contents/Java/hardware/teensy/avr/libraries/SD
Not used: /Applications/Arduino.app/Contents/Java/libraries/SD
Error compiling for board Teensy 3.6.

Any hint?

Best Regards
Erich