How large are these files? I assume they are raw, or *.wav files, in which case you won't get very long audio clips if their are 16-bit at 48kHz sample rate. Better to store the file in external memory.
In answer to your second questionL break the problem down into parts, and work on each part one at a time. So first of all, as you're new to microcontrollers, google for 'blink code'. Then look for example code for the I2S port, and output say, a sine wave, from a table and check that with a scope to make sure the I2S part is working, then finally, you could store the wav file on external memory, like a serial EEPROM. Then all your microcontroller will have to do is read form the EEPROM, into a buffer, and output to the I2S port.