I2S bus < SD!
MKR ZERO has a built-in SD port with dedicated SPI interfaces (SPI1) that allows you to play around with music files without additional hardware! For those who don't know, I2S (Inter-IC Sound) is an electrical serial bus interface standard for connecting digital audio devices.
Attention musicians: we have some news for you! We have shared two libraries for you:

• Arduino Sound Library - an easy way to play and analyze audio data with Arduino on SAM D21 based boards.
• I2S library - for using the I2S protocol on SAMD21 based boards.
Arduino® MKR Zero
The MKR ZERO gives you the performance of an Arduino Zero in a smaller form factor defined by the MKR form factor. The MKR ZERO Board is a great teaching tool to learn more about developing 32-bit applications. It has a built-in SD port with special SPI interfaces (SPI1) that allows you to play with MUSIC files without additional hardware! The board is powered by Atmel's SAMD21 MCU with a 32-bit ARM Cortex® M0+ core.
Technical specificationsMicrocontroller SAMD21 Cortex-M0+ 32bit low power ARM MCU Board Power Supply (USB/VIN) 5V Supported Battery(*) Li-Po single cell, 3.7V, 700mAh minimum DC Current for 3.3V Pin 600mA DC Current for 5V Pin 600mA Circuit Operating Voltage 3.3V Digital I/O Pins 22 PWM Pins 12 (0, 1, 2, 3, 4, 5, 6, 7, 8, 10, A3 - or 18 -, A4 -or 19) UART 1 SPI 1 I2C 1 Analog Input Pins 7 (ADC 8/10/12 bit) Analog Output Pins 1 (DAC 10 bit) External Interrupts 8 (0, 1, 4, 5, 6, 7, 8, A1 -or 16-, A2 - or 17) DC Current per I/O Pin 7 mA Flash Memory 256 KB Flash Memory for Bootloader 8 KB SRAM 32 KB EEPROM no Clock Speed 32,768 kHz (RTC), 48 MHz LED_BUILTIN 32 Full-Speed USB Device and embedded Host