NOT realtime MP3 decoder
The mp3 audio coding is not just one mathematical calculation. To decode every mp3 file you need a complete package and enough memory. That is not something for a simple arduino board.

There are libraries to decode for example a jpg file or a mp3 file. They have restrictions and you need a faster processor.
Adafruit has made a start: https://learn.adafruit.com/native-mp3-decoding-on-arduino/, but you need a powerful non-standaard arduino-compatible board. That library does not use I2S yet.

I suggest to use a Raspberry Pi.