I know the original CD tracks are included as .ogg files, and there is an image of the original CD, RTZ.gog.

Despite this, when I run the game, I get midi audio, instead of the CD tracks. I remember the music vividly from when I played it as a kid, and I just can't play it with the beepy pseudo-music I hear now.

Is there any way to play it with the CD music? I even have the original CD if I can use that somehow.

You should try using ScummVM with the files from your GOG install or from your original CD.

Info on using ScummVM with Return to Zork: http://www.thezorklibrary.com/installguides/rtz-scummvm.html

Note how to get CD audio working toward the bottom of that page. I think that you can also use the ogg tracks from GOG instead of the mp3 files mentioned in that guide. Just make sure they're named as track1.ogg, track2.ogg, etc. I haven't tested using ogg myself, but according to the ScummVM README file, it should work: https://github.com/scummvm/scummvm/blob/master/README#L1771

I think there are some areas where you you'll get midi music, and some areas where its cd audio. It depends where you area IIRC.