Voice Over not on every slide?

I often use my title/introduction slide to prep the learners - I am clear in the introduction that not all slides will have audio, but for those that do they should allow it to play until the end prior to clicking on the slide. Some of the courses I've created have software simulations incorporated, so audio on every slide is cumbersome. I also found that if the audio is just instruction, and the instructions are similar for multiple slides it seems very repetitive. 

There are a lot of opinions regarding when and how audio should be used. And then there's the research. I would recommend looking into research based answers. Here's a good article that incorporates several. If you haven't picked up the Clark/Mayer book I would highly recommend it. 

While the research doesn't go into prompts (that I've seen anyway), I'll give my dimes' worth of opinion: I think if the learner comes to a page and it is obvious that they have to answer a question by checking a box, selecting a radio button, or typing text into a field, then they don't need audio to help them figure it out. They probably did all of those things intuitively in the pages leading up to opening the course. 

@kelly and michael:

thank you for your comments! they both helped me a lot.

i will give a brief introduction in the beginning of the module about the use of audio and follow the design principles mentioned in the article michael linked to.