I would think you could select the first 30 seconds of a project you are creating, and export the entire mix - it goes to the C:\Cakewalk Content\Audio Library by default. (I subsequently moved this folder to a data drive, but wherever you have it, it would be the one I suggest you use for this.
So do the above as you create new projects, and then later, as you are previewing projects, you could simply play through these short audio clips, all located in the same folder above (or put them in a sub-folder within the above default folder).
And for your existing 100 or so projects, you could devote some measure of time over a few days, or however you wanted to give time to doing it - and go into each project and create short audio clips as described above - in effect building the audio clip preview library after the fact, but then they would all be out there for later when you need to come through and preview projects.
So - using the above approach, you can accomplish what you are seeking to do right now, and then you would be positioned thereafter because new projects would include the steps to add a preview audio clip for them.
Does that work for you?
Bob Bone