NOT realtime MP3 decoder I know that MP3 is the most popular audio format out there, but I need to be able to convert them to wav files for bother project I'm working on. It can be done with a PC but I would like to know if there is a way of decoding a MP3 file and turning it into a pcm wav file on an Arduino. I don't want to use an extra chip, as I do not plan on playing the MP3s, just converting them.
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.