@macaba I will have to look into this! Do you plan on sharing any of your work porting the audio libraries from the Teensy over to the M5Stack / ESP32?
RE: Mod to programmatically disable speaker whine/hiss.
RE: 404 Error on http://www.M5Stack.com/download/package_m5stack_index.json
I fixed the 404 error mentioned above in Arduino IDE, but only by downloading github board support repo from https://github.com/m5stack/m5stack-board-support and uploading the files in that repository to my web server. I did havr to modify the json file to makr it work by pointing the reference to the zip file at m5stack.com to the same file on my web server.
Keep in mind, this is a temporary fix only.