Back in the early nineties, I believe Sierra intended on doing talkie versions of many of their already released 256 colour VGA disk-based games, but never actually got around to doing it. (There are some other threads on here detailing some of those projects.)
In many cases, we’ve actually seen CD versions of games being advertised, but never actually produced and sold. You’ll notice that some issues of Interaction carry third-party ads for CD versions of Conquests of the Longbow (there’s a short demo on this site), Larry 5, etc. And we know the CD version of SQ5 had all the planning done, i.e. was ready to go into the sound studio, when it was canned (and what a shame!).
And as far as I know, there never was a CD version of EcoQuest 2.