The original m5stack uses an analog speaker amplifier circuit. The core 2 uses digital so you'll need to use something like https://github.com/earlephilhower/ESP8266Audio in order to store files in flash/sd/internet and play them back over i2s.
I've had great quality and volume coming through the speakers this way. I suggest you look into the github link I posted for examples, they have totally complete examples using internet sources but if you just read the examples on that github you can see how to adopt it for sd/spiffs files with ease.