Drumkit with no Sound in MS4 - MS Basic Inactive issue

Hi,

I have had issues with the standard Drumkit in MS4. When I upgraded into MS4, I have had no issues but since some days, when I open a file, all of my drumkit part doesn't sound.
In the mixer, the drumkit is assigned as "MS Basic" since there are no Muse Sound for this instrument. I don't see this as an issue.
But I have no more sound. When I go in the menu "Display" (I do not have MS4 in English the name of the menu may differ. It's between "Edition" and probably "Add"), and select "Playback Setup" (last menu under "Piano Keyboard" with P shortcut), I see that MuseScore Basic is inactive. See picture joined.

How can I activate MS Basic ?
As a music teacher this is very annoying when I have to show my scores to my students as a part is missing in the audio.

Thanks !