Reassured to find I had made a programming error and my program was trying to set the output directory to the non-existent directory "dest$" instead of the string which was the contents of dest$ ("C:\[whatever]"). Hence no files found to play, though haven't yet got straight how the files were getting output to the right directory, despite the error re dest$. I am now managing to access the recorded files with recordpad -play by hard-coding in the destination directory instead of using a variable. Not ideal. May be able to revert back to using a variable with a bit of thought, now I'm reassured that recordpad -play can work. Problem seems to have been with my program rather than with Recordpad.
Documentation didn't make it entirely clear to me - though I may have been inattentive - whether, in building a string to be sent to a command line, I needed to engineer enclosing filenames within quotes (chr$(34) + filename$ + chr$(34)). I think the current conclusion is that I don't, though I need to gain much more experience with recordpad to become proficient in using it.