There are two different approaches: either take a song with a varying tempo (not done to a click track) and you match audio/MIDI done to a click track and sync them up so they follow the track with varying tempo, or take a track done at one tempo (or varying tempo) and sync it to a steady beat at a different tempo.

I've found that generally it's more successful to "even out" an unsteady beat to a constant click. Mapping audio to a fluctuating tempo generally requires subdividing down to the beat level, which is pretty tedious. Even then, it doesn't sound that natural.

The process is pretty straight forward. Reaper is my DAW of choice, but only because it's what I'm most familiar with. I don't think you can do this in RealBand (yet) because it lacks some of the editing features. But I believe that some DAWs have tools to automate the job. My process is:
  • Figure out the "average" tempo of the song. BiaB's ACW tool is good for this.
  • Set the project's tempo to the desired tempo.
  • Import the audio into the Reaper.
  • Adjust the playback speed to 80% to increase accuracy.
  • Select the audio clip, and play it back. At the start of each measure, press "s" to slice the audio track.
  • When all the audio is sliced, set the playback tempo to 100%!
  • Turn on "Snap To Grid".
  • Go to the first measure of audio, and drag the left portion to the beginning of the measure, and let it snap it into place.
  • Hold down Ctrl+Alt and drag the right portion to the end of the measure, resizing it and snapping it into place. Ctrl+Alt allows the audio to stretch in time, but retain the same pitch. You've now resized the first measure to match the project's tempo.
  • Repeat for every measure in the song, promising yourself every couple of measures that next time you'll have them record the audio to click track.
  • Review the track at high magnification, looking for gaps that may have accidentally been added, and make sure everything is aligned as intended.
As I said, there's probably much easier ways to do this in other DAWs - this is just the method that I've used, and it works pretty well.

Once that's done, you can drop in tracks created in BiaB, and they'll sync up nicely.

-- David Cuny
My virtual singer development blog

Vocal control, you say. Never heard of it. Is that some kind of ProTools thing?