Looking at my FX on the track, I do have one plug in that we added, which is the SoftTube Saturation Knob. Aside from that, we have applied the Deesser, Compander, Doubler, Tube EQ, Sonitus Compressor, and Telephone Vocals. These are all from the FX Chain Presets folder that was created when we installed Cakewalk, which leads me to believe that the program came with them.
We've also got the Console Emulator turned on from the Pro Channel. The first time that we had the problem with the vocal track not playing after restarting Cakewalk, it turned out that the Tube EQ was the problem. We deleted it, added it back, and all was well. Then we saved, closed, reopened again, and that same track was out again. This time, deleting the Telephone Vocals got it back. It just seems like an arbitrary and painful problem to have.