Digiday reported on a study a few years back that showed 85% of people have sound off while on Facebook. So in general, super copy does the biggest lift in driving ad performance. You can also now make supers much larger since Facebook got rid of the 20% rule i.e. type can now take up more than 20% of an image, thumbnail or video (you actually always could do it within the latter.)
That all said, if you're getting a million impressions for an ad on Facebook, 150,000 people is a lot of people. And sounds can make a big difference. There have been studies that show it can be more compelling than the visuals, in general. So choose an awesome music track and keep testimonial VO! Especially as we move into the TikTok world, where sounds is nearly 100% on, it will become more and more important.