Voiceovers

In my experience, there's really no "all or nothing" rule. It really depends on the content difficulty, length, intensity of each course. The other reply here is good, you want to not have too much of the same thing for a long time in a row. I try to mix up having them read, listen, and interact with the course as they go through. I also try to build a personality for the narrator or character I'm having them listen to. Doesn't have to be long or super in-depth but making a believable person with a reason to listen to them can help engage. I don't do the voice-overs myself, as the AI voices are incredible and have a lot of versatility. It's fine to have voice overs for the first part of the courses and then just navigation after that, but try to find ways to keep it interesting. Images/Videos/Click interactions/thought provoking questions/etc... to keep the courses engaging as you go along.