This was copied from the user manual you linked:
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The analog audio signals of the currect function is output from the REC OUT jacks. When you use AUDIO SPLIT to assign a digital audio input (for example, MD/DAT OPTICAL), the analog audio signals of the assigned audio (for example, MD/ DAT ANALOG) is output.
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When 5.1CH INPUT is selected, audio signals are not output from REC OUT jacks.
Hope this helps/makes sense.