Disabling voiceover

I can't help you except to say your learners are correct. The best research shows that exact voicing over of text that appears on the screen creates huge cognitive interference. The best option for learning is pictures that are explained by voice over.

Without seeing how the course is set up across the slides, the general idea would be this:

• Create a custom button that toggles a T/F variable (example varAudioOn=F).
• You can put it on the slide master so the button is persistent.
• User clicks the button which toggles between T and F.
• On each slide you need a trigger, something like this
  • Play media file when timeline of the slide starts on the condition that AudioOn=True).
• There is some other nuance to starting and stopping the audio, but that's the essence of it.