how are you trying to render?

from the manual

Press the [.WAV] button and Band-in-a-Box will render the song arrangement to an audio wave file.
The Render to Audio File dialog box permits you to save your file as a wave file (*.wav), a Windows Media Audio
(*.wma) file, or other compressed audio formats supported by your system.
You can direct render “MIDI only” songs into high quality wave files with the included Roland VSC DXi or any other DXi or VST softsynth you have.


The MIDI parts are converted directly to audio without being rendered (recorded) in real time, usually in just a few
seconds.

You can direct render to audio as separate tracks.

If this option is
chosen, separate WAV files get written (mono or stereo) for each track (names MySong_Bass.WAV,
MySong_Drums.WAV, etc.) so you can import tracks to your favorite sequencer as audio files. For direct rendering, you can also choose whether you want the output file to be mono or stereo.

for more info see chapter 10 working with audio in the supplied band in a box pdf manual in the bb directory

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