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 is called Redbook Audio, because that is what the standard for audio cds is called. Technically, this would have to be called Orangebook because it mixes Redbook and Yellowbook (the standard for data on cds). See https://en.m.wikipedia.org/wiki/Rainbow_Books
Half life did the same, as I recall, as did many other titles.
I assume that the CD simply had two "tracks," with the audio CD "partition," if I may call it that, being the first. This allowed the game developers to play music during the game using the CD-ROM drive's hardware audio decoding (remember that small molex that went from the drive to the soundcard?) without impacting the game's performance.
These days, it's impressive to think that decoding WAV audio could at one point be a limiting factor for a CPU.
It's a balancing act thought - shipping audio as WAVs trades performance for disc usage, which can also be an issue (e.g. Titanfall shipping with 35GB of uncompressed audio, which is a pain to download).
Also, I know a fair number of games that use Vorbis for music, so not sure I would say it's very rare.
It is common for music to be compressed, that's true. But sfx, almost always you'll see wavs for those. I don't think cpu usage is a concern anymore, I believe it's about latency. That's why music is compressed. It doesn't matter if music takes 10 ms to start playing, but 10 ms of latency for a pistol shot sound is not acceptable.
Total Annihiliation also did this. It wasn't that uncommon in that era since audio compression was difficult and so a common process was to install off the CD to HDD, run completely off HDD, and then play audio off a series of audio tracks on the CD if it was present in the drive (plus maybe some oldschool DRM by checking for the CD).
Another older example: Out Run game for ZX Spectrum. You would load it from the A side of the tape and, when finished, you could play the game soundtrack on the B side while gaming. Double win: you get awesome music and you wouldn't need to rewind the tape!
I remember putting the CD for some Windows program into a CD player and being rewarded with horrible screeching sounds. A couple of weeks ago I tried it again, but only got an error message. Seems like "modern" CD players aren't as permissive.
