Thank you for taking the time to suggest a solution.
I installed both plug-ins, as you suggested, and the I rebooted my PC.
Unfortunately, the problem persists,
i.e. zero sound from the CD-ROM AND a message indicating that a plug-in is required to play CD-ROM effectively.
For what it is worth, I have also disabled my NO SCRIPT plugin, but it makes no difference.
I am attempting to run Ted Kirk's 'Theory of Music' CD-ROM: Version 3.11 (copyright 1st Sept 2009)
Any other suggestion(s) from anyone reading this is welcome.