[Dave Smith] “shows no audio in the Inspector (neither an speaker icon at top or an audio configuration panel at all) when the clip is selected in the timeline.”
What about the clip in the browser?
Did you add video only to the timeline?
[Dave Smith] “Can anyone confirm/deny if the file in a libary’s ‘Final Cut Original Media’ folder are in fact exact copies of the original files? Or does clicking, for example, the ‘Remove silent audio channels’ actually remove channels (possibly in error) during import? “
With that footage, FCPX rewraps the footage to .mov. I do not know of a case where this process would remove audio if there was audio there to begin with. I believe something else is going on.
Please check the clips in the browser, not the timeline.