tempo changes while working on a song

Hmm,
 
Measure/Beat is based on midi timing. I'm not totally sure exactly what you're trying to do - but it looks like you're trying to mark tempo based on audio, rather than midi, which is the way more natural way to do it.
 
I am supposing (anyone know for sure?) that Sonar can deduce tempo by setting measures all over the place. What is being inferred from this is the midi timing - the audio track is not changed. There is a one to one match with audio to midi timing (assuming you got the measures exactly)
 
Having set measures in sync with the audio, if you change tempo, you no longer match the audio, because the audio itself would have to either stretch or shrink to match changed tempo.
 
What can you do?
 
You could create a groove clip of the audio. This lets you specify the BPM and Key of the clip, and groove clips can be automatically time stretched/shrinked whenever tempo changes. AFAIK, this works on a clip running at a single BPM rate (Maybe someone can chip in here and clarify - I generally stay well away from this stuff). If the audio is based on implied tempo changes, you'll need to slice the audio into a new clip whenever the tempo changes, and make a new groove clip of that.
 
This isn't stuff I have done - it's just some information from the help file. I should say most folks simply record audio in reference to a midi track, say a metronome or drum track. This way, the performer knows what the beat is even if the tempo changes. And that's that - there's no modify tempo once an audio recording is done.
 
In your case, you want to change tempo with already recorded audio - and changes in tempo require changes to the pre-recorded audio.
 
 
 
 
 

Well, the primary thing to note with pre-recorded audio is; the tempo (an any implied changes within) is set in stone. You can place markers noting the measures, you can chop the pre-recorded audio into measures (or multi-measure clips of the same tempo), but; the length (and thus time) of the pre-recorded audio does not change. This fact is why you cannot change tempo with pre-recorded audio (if it is to remain the exact same length).
 
Come to think of it, I did do something similar in the past, just not in Sonar. I beat mapped an mp3 of Peter Gabriel's Sledge Hammer in Sony Acid. This played along-side of midi tracks of the song (including that famous shakuhachi). It took a little tweaking because the original drums on the audio would drift tempo, but the end result was perfectly beat-matched, and you could look at the resulting tempo map as a drummer's drift meter... To finish it off, I eq'd and tweaked that audio track so that only Peter's vocals came through, and mixed that in with the instruments. Had to show my cousin - 'How's that for match mixing?'. It was impressive, to me at least, and it showed me some of the mix elements and techniques that go into a professionally mixed track. I learned a lot that way.
 
Anyway, the only way that you can effect a tempo change that changes the absolute total time is by modifying the audio. If I understand correctly, Groove will stretch or shrink the audio to the correct time while maintaining pitch. I believe Melodyne will do this too.  
 
One trick you can use is to turn on the metronome in Sonar - that will always tell you if the midi beats are matching the beats in your pre-recorded audio. 
 
My best advice is to turn on the metronome, and set measures where needed to match the audio - this way, once you have it matched precisely, all the measure and beats for your midi tracks will be correct according to the pre-recorded audio track.

The difficulty I have is that it's required to completely map the tempo beforehand.

 
Correct.  
 
There is no way around that because you are creating a tempo within to exactly match the hard-baked tempo of the audio, drift and all.
 

When I try to fine tune the tempo map after setting tempos for basic structural points (for example), the original audio recording as well as any new audio recordings or midi instruments don't keep their position relative to the beat.

 
Correct.
 
When you change the tempo, you change the relationship between audio (in absolute sample time) and midi (measures & beats). It's not possible to get around that anywhere - the measures and beats will move around in absolute time with any change in tempo (relative to the previous tempo) - but the audio always stays the same and is grounded at the same exact absolute time and cannot change.
 

Midi in particular seems as though it should, when necessary and as an option, keep it's position with respect to measure/beat.

 
But it is, and it does. Midi will follow exactly the tempo it is given. You simply cannot change tempo without moving the location of measures and beats in absolute time - that's the very function of tempo itself.
 
If you want to note the beats and measure in the audio and tie them to an absolute (sample) position, you can use markers. Unfortunately, these will have no relation to midi music measure and beats - but at least they will show the inflection points.
 
There is the Hail-Mary method - set the project to some reasonable tempo and leave it there. Record your midi track from a real-time midi instrument performance. The notes won't be quantized, but they'll at least be at the correct time(s) relative to the audio. Not a happy scenario..

SuperG
Midi will follow exactly the tempo it is given. You simply cannot change tempo without moving the location of measures and beats in absolute time - that's the very function of tempo itself.

That's true if you're making tempo changes directly, but not if you're using Set Measure/Beat At Now. SM/BAN  is specifically designed to adjust the tempo around both existing Audio and MIDI without changing the playback timing of either. But, as the OP has discovered, it can't be undone. You can't add or remove tempo changes directly without affecting the timing of MIDI.
 
The Ref. Guide incorrectly states that setting the timebase of MIDI to Absolute will make a MIDI clip retain its absolute length, but this isn't accurate; setting the timebase of a MIDI clip to Absolute only affects its start time. I have filed both problem reports and feature requests in the past to have the Bakers to change the behavior to match the Ref. Guide so that it would be possible to make manual tempo changes (especially removing them) and have SONAR automatically re-calculate MIDI event times and durations to preserve the absolute playback timing of MIDI like SM/BAN does, but to no avail. More recently, I've asked them to at least correct the Ref. Guide to reflect reality.

As I mentioned, MIDI event start times and durations have to be modified to maintain absolute playback timing when the tempo (i.e. the ratio of timeline beats to absolute time in BPM without regard to audio/MIDI content) is changed. SM/BAN does that, and there's no reason the same algorithm couldn't be applied when a MIDI clip's timebase is set to Absolute. Obviously the writer of the Ref. Guide thought it would but didn't actually try it.

SuperG
If you set five SM's, and then modify tempo somewhere just past the third one, SM's four and five would cascade shift in place, one, two, and three would remain in place. You can't modify the audio, how do you line all that up?

Yeah, you can't line it up in Sonar, but it would be pretty simple math to change the tempo after the "3.5" SM/BAN to keep "4" and "5" in place.  Sonar already figures out the correct tempo between 3 and 3.5.

stevec
If you think about it - maintaining the start time is, IMHO, the only logical way to handle it. If you set five SM's, and then modify tempo somewhere just past the third one, SM's four and five would cascade shift in place, one, two, and three would remain in place. You can't modify the audio, how do you line all that up? You can't. Best to maintain the just start time, and at least you can set that to a specific MBT time.
 

I'm not sure what you're saying here. In the case of SM/BAN the goal is to align the timeline to a recorded performance with tempo changes that match the changing tempo of the performance. If you wan't the audio to follow a manual tempo change, you can enable Audiosnap's Autostretch mode on it.
 
So with audio, it's your choice whether the audio follows a manual tempo change or not, but with MIDI you don't have the same flexibility; it's always going to follow a manually-entered tempo change.
 
The scenario in which I most often want the Absolute time option is actually when I've made the mistake of recording a freely played MIDI performance without a click into a project that has tempo changes in it from a previous session. Then I want to delete the existing tempo changes so I can use SM/BAN to align the timeline to the MIDI, but I can't because the MIDI will follow the removal of the existing tempo changes, and the original tempo of the performance will be lost. You can add tempo changes with SM/BAN, but you can't remove them and have the absolute timing of MIDI events preserved (except by Undo the same session).