From the Beep: If you want to use the original tempo and timing you can just right click on the timeline and select Insert Beat/Measure from the dropdown menu. A window will pop up with Sonar's best guess at what on what it should be and you can edit it from there. I did this with some live tracks from years ago and although it is tedious it's pretty easy.
 
It took a little bit of fiddling around, but in the end it worked perfectly. At first, when I would try to insert beats/measures, Sonar would come back with some really crazy tempos, up in the 300s. I went to that window that lists all the tempo adjustments line by line and found that there were a bunch of tempo markers that Sonar had left in there (maybe from my previous attempts to fudge my way through), and once I deleted all those it went like butter. 
 
This really thrilled me, really, really thrilled me. My big hesitation in getting into this computer-intensive music making is that it is easier to make everything perfect than to make things a little sloppy. This is a perfect example. I hadn't really found a way to bring in the sort of subtle tempo changes that would naturally, unconsciously happen if I was playing without a computer-generated click track. I had figured I'd have to lay down the drum track first, rigid as it is, and then everything else would have to adhere to that set tempo. But this means that I can record the tonal elements first, with whatever subtle tempo transformations I want, and then mark out that tempo and fit the percussion to it afterwards. Holy crap! Transformation!
 
From Gdugan: I'm not at my DAW right now, but I think you want "Fit Improvisation". Basically, you import your audio files into Sonar, create a midi track and record 1 note for each beat of the imported audio. Then you select the midi track and choose "Fit Improvisation" which will create the tempo map for you from the midi note timing. The more accurate you are with lining up the midi notes with the imported audio beats the more accurate the tempo map will be.
 
Thanks for that. I have a few more old tunes to experiment with. I'll give that a try.

Awesome. Love a good success story... and yeah, Fit to Improv can really speed things up if you have a decent pad controller and can keep a good beat on it. Then you can always go back and make any needed corrections using the Insert/Beat Measure method or by editing the MIDI clip and repeating the fit to improv thing.

BTW one thing I do after I set up my timeline is go into the Tempo Map view and draw in a straight line from wherever the tempo is at the start of the music to the beginning of the project. That way you don't have to screw around approximating the initial tempo. So if you open a project at the default tempo of 120 but your project ends up being around 140 when the music starts you just draw a line backwards at 140 to the start of the project (or use the insert Tempo dialog I guess). Now you have an in time count in to the song.
 
It took me a few times to get that right but it does work.