I think this is as far as I can go with the current implementation. I'm not entirely satisfied and am looking into other ways of getting the desired effect.
Recording by Dylan Thomas, himself. I believe it was for a BBC broadcast, but I haven't found any reliable source of details. It is now in the public domain according to archive.org.
Picture from pixabay.com.
design and code by Z. Mahler, with thanks to Patric Muff (http://patrickmuff.ch/blog/2014/03/12/for-loop-with-delay-in-javascript/) for the solution to the delayed loop.