Hi Vijay -
Ok, I think I've duplicated your scenario—with the GB song Tempo at the default 120bpm, I imported an audio track [that was originally recorded at 124bpm], then overdubbed a new audio track against it [with the click off, naturally]. Then I went to adjust the song Tempo of 120bpm to match the original track's actual Tempo of 124bpm.
The short answer seems to be to turn off Flex & Follow Tempo on the new track [they should already be off on the imported track] and then manually adjust the song Tempo.
Flex & Follow Tempo are both found in the Edit pane [shortcut “E”]:
- Select the Track, and in the Track tab in the Edit pane deselect "Enable Flex";
- Select the Region[s], and in the Region tab in the Edit pane deselect "Follow Tempo & Pitch"
This should work. HOWEVER…if you've done any Region editing in the song, things may still get a bit messy [see below], even though the Regions should now be in sync and at the right Tempo.
If you're interested, a fuller explanation is more than a bit tweaky, so bear with me..
Initially, when I just manually changed the song tempo [without changing any track/region settings] the timing was messed up between the two tracks [which is what I assume happened to you]. GB stamps audiofiles you record in it with the current Tempo [which was 120 in this case, even though I was playing at 124] and turns on Flex & Follow Tempo automatically [found in the Edit pane—shortcut "E"]. But the imported audiofile has no timestamp [since it was not recorded in GB] —GB does not [think it] know[s] that file's original Tempo, so it doesn't [can't] automatically enable Flex & Follow Tempo. So when you manually change the song Tempo the new recording and the imported file go out of time—the new recording is time-shifted but the other track isn't.
If you turn off Flex & Follow Tempo on the new recorded track—make sure they're already off for the imported track—then the two tracks should stay in sync when you manually correct the song Tempo. BUT...if you've already cut up either/both of those tracks, apparently the Region start positions will get messed up, and you'll have to fix them manually [apparently the track lock in GB is not the same as in Logic, it's really a Freeze function, and does not help with this, so forget about it]. But if you haven't done any Region editing [cutting up and rearranging] before manually adjusting to the correct song Tempo, then everything should be in sync, after which you can edit more easily.