I had the Mac version of Quake II.
They encoded the game as an audio CD, so you could toss the disc in a boombox and listen to the soundtrack when you weren't playing. Very cool!
This was very common at the time. I suspect there was a technical reason, maybe freeing up the CPU and memory from dealing with music and offloading that to the CDROM drive.
That was standard practice in the 90s. Track 1 on the CD is the data track, and subsequent tracks are standard CD audio tracks that happen to be played in the game.
If the game didn't require accessing data on the CD after installation, you could even swap the CD with a different album and listen to what you want.
Strictly speeking the was a seperate track with the game, the music was just in the native Audio CD format.
How many hours did you play when you found out you can dismiss the "computer updated" notification?
AKA redbook audio
This was actually just how CDDA music tracks worked in almost all CD games in the late 90s. :)