I'm having a problem with midi tracks either playing the info from another track or not playing at all. I have a track where the info is visible but I can't hear the part. I had to "reassign" my soft synth to this track because it somehow lost it and was trying to play TTS... now I can see the midi info but can't hear it... as a matter of fact, many, if not all of my Soft Synth tracks have somehow lost their relationship to the plugin and have been reassigned to TTS, even stuff I recorded as a Rapture session... any thoughts?

No solutions unfortunately, but I can commiserate. I have had routings change enough that I have grown numb to verifying them. I never found out what triggers it.

So confusing, and such a drain on time. I have a cello part, for instance, when I have that track selected, and I have it soloed, what I hear is a synth part from a completely different track. I need to mix this piece and have no idea how to fix this.

at the risk of stating the obvious, did you look at your source , its instrument, port and midi channel assignments, and make sure they are not the same?

the instrument I used is correct for the soloed track, but as I said earlier, I had to reassign it... I don't know anything about a port, there is no port option or mention in any menu I pull up, I don't know how to assign the midi channel because the "create soft synth" function does that when you create the track. Midi is a mystery to me, I have to depend on the software to work correctly.

Port refers to the "road" that your midi is travelling through.  Channel refers to the "lane" on that road. It's only utilized when you have multiple synths and sources. Without getting too deep into it, multiple ports allow you to send on all 16 channels to more than 1 instrument. 2 ports would allow you to effectively have 32 midi channels, 3, 48 channels, etc. because channels 1-16 on "port 1" and "2" are discrete.But forget that for now. I think you can click on the midi tab in the pane where you wrote in yellow, and that will allow you to see what the inputs and output are for each track. If not that, click on the clips arrow right below the track name. (sorry, I'm at work right now and don't have access to Sonar)

I think when this happens it's a simple matter of not totally understanding how midi tracks work. First please read this thread so I don't have to repeat the same stuff. Thank you: 
http://forum.cakewalk.com/Issues-with-Sonar-Home-Studio-m3721656.aspx
 
 

I absolutely don't understand how midi tracks work, after 3 decades of exposure. I did create these as instrument tracks and assigned the plugins I wanted (one being a Rapture thing)... I wouldn't know what channel to switch to, but I've tried them all and still when track 11 is soloed I hear the part from track 13, and every instrument track I created has defaulted to TTS-1, in one case I don't even remember what plugin was used, so it's trash. When I started this, this song was ready to mix and press and now it's a mess. I can get the part from track 11 back by recreating the instrument track and dragging the midi parts into it, but for some reason when I do that I end up with only the left channel (and of course, in the mix I have this panned to the right)...

Read over that link and wasn't able to glean anything I can use.

I wrote out very clear instructions on setting up a midi track to play a soft synth.
Soft Synth = VST instrument. They are not plug ins, Plug ins are effects. 
You insert VST instruments from the browser.  
 
Have you read through the Documentation on how to insert and use a VST instruments? There is very good instructions, very clear to read in your spare time. There is probably some videos somewhere as well, I like reading but some prefer the videos.
 
If you check " always use local help"  then use ""HELP/ Documentation"   this will open the help document which is very easy to find what you want to know.