If CD Audio is working, then the CD is correctly mounted. Besides, some of the commentary comes from files in the game directory, and just the longer individual race introductions are on the CD.

I remember this problem back when running it natively, and it happened again under DOSbox. You should find that, if you run the game from the CD, the commentary works but the config files obviously can't be written and you then don't get the in-game samples. To get both, you need to copy over DEFAULT.CFG from the CD to the installation directory, replacing the longer existing one (the CD one just has the language). If you then create a new configuration, or just enter your name as DEFAULT, you should get all three (commentary, sound FX and CD audio). The configuration after this first run sets up a different sound driver than the one that was setup originally, but even copying those settings over to an old config doesn't seem to work. It's odd. I'm just glad I finally got it working.