Just finished a new piece yesterday. Went to start the next one, opened up a new project, and no sound. 
 
Digital piano will not play back
Rack mount synth does not play either
Midi events are being recorded successfully (digital piano in), and the volume meters ping when they are played back
Audio is recording and playing back
Soft synths are recording and playing back
Midi channel for the track reads "None". This is the default and has always been the case and always worked fine
If I change Midi Channel to 1 <default>, no playback
If I change Midi Channel to anything from 2-16 <default>, plays back fine
 
What should I look at next? 

Depending on if your midi is real midi ( 5 pin) or USB you need to go into options and make sure the device output is on the list and checked. 
It will then be the output option for the track your trying to play. 

All midi tracks need to have an input and a output or there is no sound. 
We normally set the output to a soft synth. 
In your case it will be your audio interfaces MIDI ports, or the devices USB connection. 

The input and output are not the issue, but the Midi channel setting. "None" has always been the default and worked fine for years. All of a sudden now I have to set it to a Midi channel other than 1. 

I can't think of a scenario that would explain this that doesn't involve changing both the channel that your controller sends on and the channels that external synths receive. Soft non-multitimbral soft synths won't have this problem because they are usually programmed to respond to any channel.
 
But I would look at it one step at a time. What channel is recorded from your controller? And is the digital piano your
controller?

Yes, the digital piano is the controller. If I record with the default settings, as I always have, the output channel for the recorded events shows as 1 in the Event List view. If I then explicitly change an individual event to channel 3, just that event plays. If I change the channel of the entire track to 3, the entire track plays. It's a very clean repro: Change the channel to anything between 3 and 16 and it plays. None, 1 or 2 does not. (Yesterday, channel 2 would play. This morning it does not.) In all cases, Midi recording works fine, and no matter whether I hear playback, the volume indicators move for all notes.

And it's both your digital piano and other rack synth(s) that respond only on channels 3-16? Does the MidiSport have an event filtering/routing capability?

Thank you for asking those questions. It caused me to isolate each synth (rack and piano). I discovered there was no sound from the rack synth at all on any channel. I found that the button on the Midisport that toggles between USB/Merge and THRU was set to THRU (a cat knocked it, possibly?). Switching that to USB/Merge fixed the rack synth, which now operates correctly on all channels.
 
This meant the Midisport is working via the 5-pin connections used by the rack synth. The piano is connected via the firewire-y looking connector, whatever that is, and since it is ALMOST entirely working (input is fine, output works on 3-16) I suspected the culprit was the piano itself. I exited Sonar, rebooted the piano, went back into my project, and now everything is working fine.
 
This Yamaha piano I just bought makes all my piano tracks sound way better than before, and it's a joy to play, but it's very finicky when it comes to Midi.
 
Thanks everyone for your help!

Cool. Good work investigating. Glad you got it figgered.