Your last post triggered me to try something different.
I began with an all audio track project that I’d edited a few months ago on this computer. Upon loading it up, It played fine…
THEN I imported the midi file I’m trying to edit INTO a copy of THAT project and – lo and behold – it plays…NO fiddling, no finagling, no fuss, no muss…even assigned the instruments in Halion “correctly”, etc.
So there’s SOME setting (or combination of settings) cleverly hidden by the Cubase engineers (could they BE more obtuse?) that I’m not finding in the OTHER project I created from scratch.
The original process I went through was to choose one of the basic templates – Acoustic Guitar and Vocals – import the MIDI and delete the guitar and vocal instrument tracks and that’s the version that STILL won’t make a sound…
Ok, I found the VST Connections window (it was hidden way to the right of the visible panel - the curse of 2 monitors), I set the outputs in the “From Scratch Version” to stereo left and right and it STILL WON’T PLAY in the “from scratch” version. Maybe I have to close and reopen/reactivate it…who knows
BUT – who cares!
I can hear playback in the “start with a working project” version so I CAN do the editing of the MIDI drum track that I need to do.
I only entered MIDI-world to grab a drum track from a downloaded song to export into my Beat Buddy pedal – I NEVER record midi…always analog audio…
Thanks for your help… Even though I still have NO IDEA why the “from scratch” version won’t play, your suggestions DID trigger a work around for me… 
PS: This desktop is my day-to-day stuff and Video editing computer. In my studio, I have an HP laptop that is connected to 2 Roland Octa-Capture units… I mix in analog – Allen & Heath board, outboard efx such as SPX-90, Quadraverb, DSP128, Alesis compression, MicroVerb, etc. then back into a digital recorder or 2 mix tracks in Cubase. Since emerging from the audio learning curve a few years back I have no problems with Cubase in the analog realm. 
PPS: AH! Found it! When I loaded the “From Scratch” project and set the Stereo Output channels to my sound card in the VST Connections panel, the Halion instrument assignments weren’t there any more because I forgot to save them. So, I assigned instruments and VOILA! It plays… Sometimes “flexibility” means way too many moving parts… 