Fair enough. I appreciate the detailed explanation. It seems that while the randomness of what solo button is lit up, the appropriate track (and only that track) does play as expected, so I shall live with it.
 
Thanks again. Glad I can get this off my list of issues.

Re-read what I wrote earlier; the logic is very simple and explains everything you're seeing. There's nothing random about it. Solo a MIDI track and all audio outputs from the synth are soloed. Solo an audio output, and all MIDI inputs to the synth are soloed. The synth could be routing any or all MIDI channels to any or all instruments, and any or all instruments to any or all audio outputs. The way SONAR handles soloing accomodates any routing. You might be routing MIDI tracks to instruments to audio tracks on a one-to-one-to-one basis, but that's not always the case.