Thanks Matt and Graham for your additional input.

The spurious grey-stripe is now identified as just a graphical glitch probably generated by a long song/tune with particularly busy content/structure and key signature changes that altogether might confuse BIAB now and then.

This might also bear out your observations Graham?

It doesn't affect the work, which plays normally underneath.

In this regard, the initial sound-quality issue seemed to disappear though I can't account definitively for that. I developed the piece a little further which included removing a bar or two here and adding a couple there along with one or two refinements, and then it did not have the sound issue any more and I couldn't then replicate it either. It may still be around all the same however for the right-wrong set of circumstances.

So now I don't have a problem with the grey stripe appearing as it is just a presentation hiccup across a few bars, and is likely only to appear in the occasional more-complex structure anyway.

Though, by deselecting "New line for every section" in Options > Display > Preferences the glitch can be made to disappear if you want.

My thanks and appreciation go to Kent for untangling all of this!

Howard.