You're right, there is no limit to the number of Kontakt instruments beyond maxing out your computer's resources.
 
The only way to see this effect is to have a single instance of Kontakt hosting multiple instruments that share a common channel assignment (either the same channel number or set to Omni).
 
If an instrument doesn't get any MIDI events, it won't produce any audio. With no audio, it stands to reason that audio routing cannot be the problem, whether we're talking about inside Kontakt or within SONAR.
 
So if you're hearing tubas when the trumpet track is soloed, the tuba track must be getting data. And I'm pretty sure the only way for that to happen is to have both tubas and trumpets on the same MIDI channel, or on Omni. Double-check not only the channel assignments within Kontakt, but the track routing as well (make sure it's set to a specific channel number, not "None").

brundlefly
Did you by any chance start with Kontakt inserted as a Simple Instrument, and then clone the track? SONAR really shouldn't allow cloning Instrument tracks because they are by definition a combination of one MIDI track and one audio track with a soft synth linking them.  The GUI allows it, but cloning an Instrument track does not clone the synth instance as it should, and it creates hidden linkages between track controls that can produce unexpected results like this.

 
Actually, yes, I do believe I did this after watching a youTube tutorial. That has to be it! Genius. I know to never do that again.

I think I may actually have spoken too soon. I just started a new project to test and the problem persists. Here is what I did:
 

1. New Project, with one instance of Kontakt 5. Instance has two instruments in it, Violin and Cello from the String Ensemble NI product line.
2. Created two sets of the following. NOT cloned, but created manually and separately:
   1. Audio channel - routed correctly in Kontakt
   2. Midi channel for note input
   3. Midi channel to use with my Akai LPD8 for articulations and dynamics

 
So, two completely manual setups. Here is what happens when I select "solo" of the various channels/tracks:

• Solo Audio (Violin): Solo activates for audio violin, midi violin, control violin, midi cello, control cello
• Solo MIDI (Violin): Solo activates audio violin, midi violin, audio cello
• Solo Control (Violin): Solo activates audio violin, control violin, audio cello
• Solo Audio (Cello): Solo activates midi violin, control violin, audio cello, midi cello, control cello
• Solo Midi (Cello): Solo activates audio violin, audio cello, midi cello
• Solo Control (Cello): Solo activates audio violin, audio cello, control cello

There appears to be no rational rhyme or reason to why these assignments activate this way when I solo a track. 
 
I went one step further and removed the audio assignments in Kontakt, then recreated them. Issue persists.
 
To be clear once more, I did not clone the tracks; I created them all manually.
 
This is incredibly frustrating.

How are you creating the Kontakt instance? If you aren't already, try simply adding Kontakt to the Synth Rack and pointing a midi track to it. When the dialog to add Kontakt to the synth rack comes up, you should be able to create an audio track for each output available in Kontakt. I believe there are 3 versions of the Kontakt VST: 1 Stereo Track, 8 Tracks, 16 Tracks. I always use the 1 Stereo Track version and bounce each instrument to audio prior to mixing.
 
You can then add instruments to Kontakt and add additional midi tracks to drive them. Soloing the midi track solos only the related instrument.
 
This is how I do it and have never encountered what you describe.
 
Regards,
Dan

1. I use the Synth rack and add an instance of Kontakt.
2. I choose which version - usually the 8 track.
3. I route the outputs in Kontakt
4. I add an instrument in Kontakt
5. I add one audio and one midi track and do the assignments to relate them to the instrument
6. I repeat for another instrument
7. Mayhem ensues as described above

 
 

obiwanshinobi30

1. I use the Synth rack and add an instance of Kontakt.
2. I choose which version - usually the 8 track.
3. I route the outputs in Kontakt
4. I add an instrument in Kontakt
5. I add one audio and one midi track and do the assignments to relate them to the instrument
6. I repeat for another instrument
7. Mayhem ensues as described above

 
 

Bizarre. I'd suggest sending a simple project like you just created to Cakewalk support for analysis.
 
Dan
 

I did, Dan. Hopefully they will have an answer. Thanks.

It's normal that soloing any MIDI track will solo all audio output tracks, and soloing any Audio output track will solo all MIDI inputs. This is necessary because SONAR has no way of 'knowing' how MIDI and audio are routed inside the synth. So it needs to solo all MIDI sources to ensure a soloed audio output doesn't go silent (or lose parts) and vice versa.
 
What would be a problem is if soloing an audio or MIDI track for one instrument soloed both audio and MIDI tracks for some other instrument, but I don't see that happening in any of the examples given.
 
So long as you aren't hearing another instrument soloed, this is working as intended.

brundlefly
It's normal that soloing any MIDI track will solo all audio output tracks, and soloing any Audio output track will solo all MIDI inputs. This is necessary because SONAR has no way of 'knowing' how MIDI and audio are routed inside the synth. So it needs to solo all MIDI sources to ensure a soloed audio output doesn't go silent (or lose parts) and vice versa.
 
What would be a problem is if soloing an audio or MIDI track for one instrument soloed both audio and MIDI tracks for some other instrument, but I don't see that happening in any of the examples given.
 
So long as you aren't hearing another instrument soloed, this is working as intended.

Seems quite strange to me. My confusion really lies in two things: first, your explanation doesn't explain the randomness from which the soloing occurs. Why, for example, would soloing the midi violin activate the solo on audio violin, midi violin, audio cello, and nothing else? Why the audio cello and not also the midi cello? If what you say it true - soloing any midi track will solo all audio output tracks - then why did the following not also solo the audio cello?

• Solo Audio (Violin): Solo activates for audio violin, midi violin, control violin, midi cello, control cello

 
Second, it also doesn't explain why when I added EQ to one track, it ended up cancelling out parts of the second based on the amount of high-pass filtering I was doing. (Just one example.)
 
Not trying to be a pain; I'm really trying to understand the issue so I can either correct it moving forward, find a workaround for it, or get Cakewalk to fix it. :)
 
 

I'm at my DAW today, and want to try to emulate this.  When you Solo and instrument, where are you clicking Solo?
The Sonar MIDI track?
The Sonar Audio track?
The instrument inside Kontakt?
The big 'S' button in the Sonar Transport Module?