Regardless of the version or OS your BIAB happens to be, you have to convert the audio residing on the Audio Channel to A Performance Track and move it to any other BIAB Mixer Channel. For instance, if the style that's loaded for your song doesn't have an instrument loaded on the Strings, Melody or Soloist tracks and you don't plan to add tracks to any of them, move the Audio to one of those tracks - Example Move the audio to the Melody Track.
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To move Audio to another BIAB Mixer Channel:
At the top of the Chord Sheet on the Main Page, select Audio and from the Pull Down Menu select Move Audio to Performance Track. Note: You'll be prompted to save your project before any track can be converted to a Performance Track. In Windows version, the other tracks besides the Audio Channel can access this command from the individual channels drop down menu when you right click onto the channel and select Track Actions.

If you have a 2021 Windows or Mac version of BIAB, you can also move Audio to one of the 16 new Utility Tracks or you can record Audio directly onto any of the Utility Tracks.
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To record audio onto the Audio Channel once the existing audio has been moved to another track, select the Record Audio Button located in the Transport Tool Bar on the Main Page.
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As an FYI, you wouldn't likely want to do this, but in the "Record Audio" menu page, if the 'Overdub underlying audio' box is checked, you can record new audio that's merged with any existing audio that's already residing on the Audio Channel.

BIAB 2025:RB 2025, Latest builds: Dell Optiplex 7040 Desktop; Windows-10-64 bit, Intel Core i7-6700 3.4GHz CPU and 16 GB Ram Memory.