With BB, you can get one track fixed - the audio track.

You mute the tracks that you dont want fixed, then Freeze the tracks, which will make the audio track contain the tracks that weren't muted. Then you can unfreeze it, and keep the audio track that contains the track that you didnt want to change.
I realize that this isn't the same thing as having a track that never changes.
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With RealBand, you can get exact arrangements that won't change

Have Fun!
Peter Gannon
PG Music Inc.