I found the answer. Hooray!
I was running the Caffeine background App which intermittently does a key press of a rarely used key (The F15 'key up' event) on the keyboard to stop the display turning off and stops you needing to login again after a timeout. This key is fine for most applications but PowerPoint doesn't like the key and gives an error beep to express its dissatisfaction.
The solution is to invoke Caffeine with the command option
caffeine64.exe -stes
Can I request that Powerpoint ignore function keys that it doesn't understand please?
In searching for a solution I found that this question comes up time and again on various forums including this one and this is the first time anyone has described the solution. All of thise questions are now closed. Could someone at Microsoft reopen them and add this answer so that other people with this problem can find the correct solution?
If you can do this I will post the links to these questions.