You can draw a box the same size and put it on top of the control background.
It will disappear when the video plays.
If you are only playing a sound, you can eliminate the visualization portion
of the player simply by clicking on the top and dragging down as if you were
resizing the control.
Thanks for the advice. It works.
Indeed, I found that at least a background color during playback in
slideshow mode can be changed in WMP options. This setting keeps when a WMP
object is inserted in PP presentation.