Sonar for Remixing

I suggest grabbing a Sonar Power book.
 
Basically, the standard way to grab the tempo for a pre-recorded track is to use a midi device and tap the tempo into a midi track as you listen to the primary track. Then extract the tempo from the midi track.
 
One of the members created a tool for slurring the tempo changes over time to try to make this type of tempo extraction more precise, but the first step is to use this method for extracting the tempo.
 
There are two other methods I sometimes use...
1. Open the Tempo View and display the tempo events on the right. Then click the plus button and you can tap in the tempo. If you audition the track before you start playback you can tap along and get the tempo for the project and just set the tempo for the whole project to that setting.
2. You can set the current beats as you listen by hitting Alt+M I think?? someone help me out here. But basically, you specify down-beats one at a time and you end up with the project tempos.
 
Personally, recording the midi track where you tap along works WAY better than anything else in my experience. Keep in mind that when you have sharp changes in tempo on tempo synced plugins you can get strange behavior like clicks or pops. Here, I'm thinking of the Vocal Channel Plugin that comes with Producer.

If the original recording didn't change tempo, it's pretty easy even without tools. Count off the number of measures from the beginning to wherever you run out of patience, then adjust the tempo for the same number of measures to cover that part of the music. I did a remix of Mark Longworth's Magic Spell and not only added parts, but kept some of his tracks as well...you won't hear anything out of sync.