For pins 12-15...
	PIN_FUNC_SELECT(GPIO_PIN_MUX_REG[xx], PIN_FUNC_GPIO);


SD card:
	https://github.com/espressif/arduino-esp32/tree/master/libraries/SD


Sound:
	https://www.esp32.com/viewtopic.php?t=3693
	for (z=0 ; z<size ; z++)
	 {
	      dac_output_voltage (DAC_CHANNEL_1,buffer[z]); 
	      delayMicroseconds(125);        // 8kbit/s period
	 }