I was trying hard to get this working yesterday and used the sample from http://forum.m5stack.com/topic/143/lesson-6-1-speaker-mp3-player/2
Tried to reconfigure the dac output by changing the following according to the information here https://docs.m5stack.com/#/en/base/node_base
out = new AudioOutputI2S(0,0);
//#define I2S_WS 13 //LRCLK - Frame clock
//#define I2S_BCK 5 //BCLK - Bit clock
//#define I2S_IN 2 //DACDAT - Serial data input.
//SetPinout(int bclk, int wclk, int dout)
out->SetPinout(I2S_BCK, I2S_WS, I2S_IN);
mp3 = new AudioGeneratorMP3();
playing = true;
but no luck -> the file still got played through the internal speaker.
Maybe somebody has a clue how to proceed