Back when I was working at Cinemaware in the late 1980’s, I was given the task of adding CD quality audio to “Defender of the Crown” for the PC (Mirrorsoft publisher). It was already a successful game, but video games didn’t have high quality audio back then, so it was a neat thing to do. Dave Riordan took care of creating the CD quality music and voices, while I added hooks to the code to play the music and narration off a CD-ROM. Note that the version I created (shown at a conference) was not a mixed mode CD-ROM, as the code did not reside on the CD-ROM — I don’t know if Mirrorsoft later placed the code on the CD-ROM when it was published.
I had no idea until recently that there was a 1989 “New Scientist” magazine article about Mirrorsoft’s “Defender of the Crown” for CD-ROM. I don’t know if the reviewer was talking about the specific Hitachi CD-ROM version with CD quality audio I helped create, but it might have been. The review is not especially flattering, but that may be because the reviewer calls himself “a games hater”
Wikipedia states, “The earliest examples of Mixed Mode CD audio in video games include the TurboGrafx-CD RPG franchises Tengai Makyo, composed by Ryuichi Sakamoto from 1989 and the Ys series, composed by Yuzo Koshiro and Mieko Ishikawa and arranged by Ryo Yonemitsu in 1989.” But I worked on “Defender of the Crown” with CD quality audio in 1988, so it’s possible that Mirrorsoft’s version was the first video game to include CD quality audio.