Module developer fewieden kindly picked this up. Apparently changes in MMM over time caused some incompatibility.
He has now released version 2.0.2 ; I can confirm that the problem is solved.

Pi4 with latest Raspberry OS, latest MM version, official 7" touch screen. Official power adaptor.
Modules:
Alert
MMM-AlarmClock with minimal entries as shown on Github and this :
old thread

After touching the screen, the alert clears but the sound file continues to play 3 times. I want it to stop!

Have tried a fresh OS and MM install but the same result.

Is this expected behaviour? If so I’ll ask the developer how to change it. If not, what might I have done wrong?

Any comments welcome.
Thanks.