"Originally pressed on the same disc as the game, Quake was otherwise commercially unavailable until its release on vinyl in September 2020"
Soundtrack: https://www.youtube.com/watch?v=zbLttdCOgCM
I guess there was a point when CD-ROM was the target format anyway but game content hadn't necessarily grown to fill it yet. Having the CD drive play the audio directly (it could be directly connected to the sound card) with no decompression or software processing required probably saved some CPU cycles as well.
I think at least half a dozen games I had around the time did that. Of course most of the soundtracks wouldn't be interesting enough on their own.
I know both DMA and PIO-only SCSI HBAs existed at the time, but I'm not sure about the proprietary interfaces that were more common for consumer CD-ROM drives.
Panasonic (MKE) was pure pooled PIO and didnt even support Interrupts. Sony and Mitsumi do have DREQ/DACK pins so most likely did ISA 8bit DMA. ISA DMA is pretty terrible and slow (4.7MHz clock, steals CPU cycles for the duration of the byte transfer), barely enough for 2x CDROM. IDE PIO is better on 286 and up than ISA DMA thanks to nice dedicated REP INSW. Taking into account ISA limits transferring one CD sector should take no less than ~700-1000us.
While ISA SCSI and VLB controllers could do DMA, they rarely did with ISA 16MB memory limit and VLB compatibility worries. The one VLB IDE controller dlaiming to do DMA, Promise PDC20630, does MWDMA2 to the drive but with CPU manually pooling IO pins on the other side, total waste https://www.vogons.org/viewtopic.php?p=1323789
TLDR: Talk of enabling DMA for your HDD and maybe even CDROM became common only somewhere around 1996, a good year into Intel shipping busmastering IDE controller in PIIX on Pentium boards (430FX and up). You had to put devices on separate IDE channels and hope everything is compatible and works.
The music was made interesting by the sheer novelty of being able to listen to it that way, and get a bit of the game to enjoy even after the parents decided computer time is over =D
The interwebs also mention G-Police, which I remember playing, but I have no recollection about the soundtrack.