Thanks Derek. Howver the complete story is that insert-markers worked on other audio tracks in the same sequenece - Even a new music file didn't fix the problem. Later things corrected on their own.
I recall back in FCP 3 reading online about native FCP files becoming corrupt. This was in the context of folks wanting to fix their master without re-editing or other reasons. There were actually examples of code fixes you could do with a the right editor to open up the FTC files. At the time I was able to fix my problem by opening a new sequence, performing the edit work there, then copy/paste in to the master sequence. Back then, and maybe now too, I have a suspicion that a file corruption may have affected that particular point in the timeline.
Frank