I have a project with audio tracks, instrument tracks, and plain Midi tracks in it. For Audio and instrument tracks, the output is set to one of a handful of buses. These buses in turn output to my Master bus. The straight Midi tracks' outputs are set either to my Midi card or else, in the case of my keyboard, the actual keyboard output. The problem I'm having is that if I solo one of the audio tracks, I'm still hearing the Midi tracks as well. I'm confused how I'm supposed to set up the Midi tracks so that I can solo and audio track and the Midi tracks are not heard. The Midi track output doesn't offer a bus or Master or anything that I can output to. I'm figuring it's something else. I don't want to bounce these audio tracks to audio yet, as I'm not done with them. So, what am I doing wrong?

Do you send the output of the MIDI instruments to audio tracks in Sonar?
 
1-I have MIDI tracks that send MIDI to a keyboard.  Then the keyboard audio outputs go into my interface and are routed to audio tracks in Sonar.  Solo works fine.
2-If I turn on the keyboard speakers, I will hear the MIDI playing through the keyboard speakers because that audio is independent of (and outside of) Sonar. 
 
I would highly recommend that you bring all audio through Sonar so you can mix it and control it centrally.
 
Note: In the case of virtual instruments, Sonar is smart enough to mute or solo the MIDI when you mute or solo the associated audio track.  If you route a MIDI track to an audio track, Sonar does not really realize these are connected.
 
I agree with you that solo should mute ALL other tracks.

Well yes, that would be easy, but when I go to select the output for a Midi track, the only options are all the soft synth tracks I've add to the browser, and the Midi card. How do I output a Midi track to an audio track?
 
(I understand what you mean about turning off keyboard speakers, and I do have that set up correctly, as I can mute the keyboard's track from Sonar just fine.) 

jkoseattle
How do I output a Midi track to an audio track?

Why do you want to output a MIDI track to an audio track? If it's to control a processor or something, you need to select the MIDI track output in the effect's VST header.

jkoseattle
The straight Midi tracks' outputs are set either to my Midi card or else, in the case of my keyboard, the actual keyboard output.

By "MIDI card" do you mean your MIDISport 2x2 interface or do you mean a soundcard's "wavetable" synth? And is the keyboard a keyboard synth or just a controller? I gather it's connected by USB, not via the MIDISport; is that right? Are the outputs of your MIDISport connected to some external sound module that you're direct monitoring?
 
I hadn't thought about it for a long time because I input-monitor the audio output of my external sound modules and keyboard synths through SONAR so the audio gets cut off by soloing another track regardless of whether MIDI is still sent, But I just checked, and the the MIDI activity light on my interface goes dark when I solo an audio track. So even a direct monitored hardware synth would be muted.
 
So I'm not quite sure how you would continue to hear any MIDI-driven synths - software or hardware - when soloing an audio track, unless possibly you have a direct-monitored keyboard workstation with an onboard sequencer that's playing its own internal MIDI sequence, synced to SONAR. Seems unlikely, but it's the only setup I can imagine that fits the symptoms.
 
In any case, it will help if you describe your hardware setup a little better. You're playing a little fast and loose with the terminology, which makes it difficult to understand.
 
You've been around here long enough, I would have thought you'd have this all worked out by now. Is this something that just started happening recently?

Thanks for the explanation. Your implication that this should be obvious made me look at things again, and I've better refined the problem. If I solo an audio TRACK, all is fine. If I solo the audio track's BUS, then the Midi tracks still play. The audio track's bus routes to the Master bus. If I mute the Master bus, the Midi tracks still play. Those Midi tracks aren't even going to Master. I'm sure this is something really stupid I ought to know. 
 
Anyway, to explain better, here is the rundown. I have two Midi tracks. One is from an external piano keyboard which is connected via USB to the computer, and there's a Midi output especially for it called DigitalPiano1 which as I recall got installed with a driver. I also have a rack mount synth which is connected via a MidiSport 2x2. The output for that is MidiSport 2x2. These are the two tracks that still play when I solo a Bus. In addition, I have a couple dozen soft synth tracks from EastWest that I'm having no problems with. Then I have a few audio tracks for vocals. 
 
The audio output of both the rack mount synth and the keyboard go directly to my external mixer. Turning the volume down on those channels mutes them, even when the entire mix is playing. So that audio output is bypassing Sonar entirely. Which is why I'm hearing them when I solo a bus.
 
I think what's changed is that I'm routing through buses more than I used to, and I must be missing some aspect of that. The only output options for the Midi tracks are the two Midi outputs (DigitalPiano1 and MidiSport 2x2) and then all the EastWest instruments. 
 
 

Good job. It sounds like you've identified the issue yourself: Soloing buses mutes audio output of other buses and audio tracks but does not affect MIDI, whereas muting tracks does.
 
I think that's working as intended, because you could have an audio track receiving input from a hardware synth that outputs to to the soloed bus. You wouldn't want that audio source to go silent becasue it's MIDI track got muted by soloing that bus. And SONAR doesn't have any facility for knowing which MIDI tracks are associated with which audio inputs; some other DAWs allow you to create this association.
 
I think the long-term solution for this in your case will be to get an interface with enough audio inputs to accomodate input monitoring your hardware synth outputs.