The sound is whatever sound engine you have set as the default midi sound engine in Band-in-a-Box. If BiaB is using a stereo patch the audio output is stereo while if BiaB is using a mono patch the audio output will be mono. I haven't found a way to convert a file from stereo to mono in Band-in-a-Box. RealBand has commands to convert two mono tracks to stereo or a stereo to two mono tracks but nothing to combine stereo to mono.

I'm not saying this is the only way but I'd export the bass track knowing it will be stereo in this case and then use a wave editor to convert stereo to mono.

I use Audacity for tasks like that. There is a command just to perform that task

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Audacity stereo to mono command.
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RealBand track options.
Jim Fogle - 2025 BiaB (Build 1128) RB (Build 5) - Ultra+ PAK
DAWs: Cakewalk Sonar - Standalone: Zoom MRS-8
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