King Conga
I simply route the MAIN OUTS to ANALOG Ch.s 5&6 on my PreSonus 1818VSL.

 
The only way you're going to hear output from a keyboard synth without an audio track in SONAR in this case is to enable direct monitoring via a physical switch on the interface or a software switch in the interface console software.
 
With Cakewalk for DOS, you wouldn't have had an audio interface, and would have been routing the audio ouput of your keyboard direct to your monitoring system.

Does your keyboard have phones out? Try to listen to your keyboard's phone out using headphone. If it does not make any sound, then the problem is in the MIDI OUT path of your sonar (interface) to your keyboard. If it makes sound, then there is no problem with the MIDI setup, we can focus on the Presonus.
 
From what I see, I suspect you need to check Presonus driver interface to make sure the channel 5 & 6 is not muted, and have their faders all the way up. Othewise, you need to activate direct monitoring in Sonar audio track like Brundlefly mentioned above to be able to hear the sound.

Thanks James. I do have a hdphn OUT and it DOES produce sound, as do the actual MAIN OUTS that are sending good signal to the interface Analog INs, and both MIDI and 1/4" cables are good, and nothing is muted.

 OP- In fact, the clipping indicators for my analog channels on my interface will turn red but they just won't sound.
 
Then now- Thanks James. I do have a hdphn OUT and it DOES produce sound, as do the actual MAIN OUTS that are sending good signal to the interface Analog INs, and both MIDI and 1/4" cables are good, and nothing is muted.
 
Why I deleted my posts. 
 

Please forgive me for jumping in at the end - I did read some of the posts - rather lightly, I confess in advance.
 
Is it possible that your external keyboard has the MIDI parameter Local set to 'Off'?  That is something us keyboard players routinely did/do, to stop a doubling effect of any keyboard notes triggering the internal keyboard's sound, then the MIDI data coming back into the keyboard ALSO triggering a 2nd note, hence doubling.
 
SO - just wondering if perhaps Local control is off.
 
Forgive me if I missed the mark on the above, 
 
Bob Bone
 

Yes, Bob.  Thanks, the Local IS Off.  I notice you have the same 1818VSL I do.  Do you have, or use hardware monitoring?  I tried to look for that setting on this device but couldn't find any.

I monitor only through Sonar.
 
Ages ago I had to do some weird things to the Presonus but that was in that software mixer program that they did away with.
 
I now trigger all keys from soft synths, using 2 midi controllers.
 
Bob Bone

King Conga, it seems you missed my post #21. If you don't want to echo audio through a track in SONAR, you will need to enable direct monitoring via your interface.
 
EDIT: Looks like you are working on it. I Googled a bit but didn't find anything definitive. Perhaps something in this article will help:
 
https://support.presonus.com/hc/en-us/articles/210044383-AudioBox-1818VSL-How-do-I-create-a-monitor-mix-for-my-headphones-
 
I considered getting an 1818VSL once, but passed because it's low-latency performance was not great (considerable hardware/firmware latency on top of the buffers).

Well, Lordy Brundlefly, you're pretty handy to have around. I think I'll keep you.  I was wondering why my hdphn wasn't working.  Do you think that may be why I'm not hearing MIDI? At the very least, this will help.
Thanks,
KC

King Conga
Well, Lordy Brundlefly, you're pretty handy to have around. I think I'll keep you.  I was wondering why my hdphn wasn't working.  Do you think that may be why I'm not hearing MIDI? At the very least, this will help.
Thanks,
KC

MIDI has no sound! You need an audio track to hear it...  hehehe  Just kidding,