That's not an easter egg. That's just a dual format CD - you can burn them yourself really easily (I used to write these as MP3 CDs so systems that didn't support MP3 could still play a subset of the tracks).
It was a really common practice to put in game music as audio tracks on game CDs. Dreamcast GDs also work a similar way where you can play the game audio in a regular CD player despite the data track being formatted differently to the ISO standard.
Yup, I was amazed when I put my Moto Racer [0] game CD into my CD player and all the background songs started playing! I was just, in my naiveté, trying to see if the CD player would be able to convert the game binary data into music.
It was a really common practice to put in game music as audio tracks on game CDs. Dreamcast GDs also work a similar way where you can play the game audio in a regular CD player despite the data track being formatted differently to the ISO standard.