I believe you're (Haggis) talking about the pop-up volume bar in display view as shown in the screenshot? Is that correct? If not, then what markf responded in his post.
If for the Display View however, there is no built-in way to do what you want. That said, if you're willing to dig into the skin XML file (not the parts Mark mentioned, but essentially suppressing the OSD and PROGRESS BAR skin XML sections), and then ensure your Display View background and OSD text color are the same, you can kind of get there. But even then, if you use a skin that is included with MC standard install, those changes will get overwritten each time you update MC, so you'd have to make sure you save it off into a new folder with a new skin name to prevent that. If you are interested in jumping through the hoops to pursue an imperfect solution, let me know and I can follow up with an outline of the steps required.
Otherwise, it probably makes more sense to see if Matt is willing to implement a new setting that turns off the OSD pop-up altogether for Display View. I too would prefer to not have the OSD pop-up for audio and video, but would like to keep it for image slideshows. So if a new setting can be created that toggles the OSD popup for audio, images, and video independently, count me in.