It seems to be a common glitch in the MM-15 program.  I had the same problem when I first upgraded to MM-15 Premium.  I had it again after the program checked for updates.

I've found the best thing to do is to delete and then reload the soundpools.

First, open MM-15.  Then, go to File > Settings > Program Settings OR simply hit the 'Y' key.  Make sure the "Use Installed Soundpool Even After Reset" box is not checked and click on the "Reset Soundpool" button.

The soundpool should now be empty.

Close the pop-up by clicking OK. Click on "Add Styles"; go to folder...C:\Program Files\MAGIX\Basics_15...and click OK.  It should then load all of the soundpools contained in the Basic_15 folder.