Just for the record, I found out (by reading THROUGH the manual instead of trying to find the topic with the index or TOC) that one has to change one's CAPTURE SETTINGS in the A/V SETUP dialog prior to capture. From there it's a matter of recapturing the same clips using AUDIO ONLY.

In case anyone else is looking for it.

I just shot a documentary using a damaged XL1, it's first two inputs were not operational so we had to use 12-bit mode to use the second pair. The audio was fed line-level from my PDX10 which was used as preamp and dual system just in case. As it turns out, 12 bit sounds well enough for interviews, we had no need to use and sync the PDX10 tapes.