Sorry to hear about your problem. I appreciate your frustration. What you're describing sounds a lot like problems with the database that is used to manage the soundpools. It's a problem that has been described by other users before, but, so far, it has never really become clear under which circumstances this issue will occur, and why it seems to happen repeatedly for some users, but doesn't seem to happen for others.
Here's a practical approach to try and resolve this: We're going to completely reset the soundpool database, which is done by manually deleting the database file in which it is managed. This will not result in any actual soundpool/loop files being deleted. It's just a reset of the database to eliminate potentially incorrect links to the loops or other faulty database entries. In order to do this, please locate the database file "SoundPoolDB.dat" in the following location on your hard drive:
(In order to be able to navigate there and find this file in the first place, you might need to reconfigure your Windows Explorer to show hidden folders and files first. In order to do this, open the "Organize" menu in the Windows Explorer window and go to "Folder and search options". Switch to the "View" tab in the dialogue that will come up, find the option for hidden files/folders in the list and tick the option to "Show hidden files, folders, and drives". Confirm your changes via OK and try again.)
Delete the file (while Music Maker is not running), and then restart Music Maker. The "Loops" section will automatically be populated with available soundpools again. Should any soundpools that are installed in the defaut location not be listed, please open the program settings ("P" key) and trigger the rescanning of the soundpools via the "Rescan Soundpools" button in the "General options" tab. If you need to add MAGIX soundpools that are installed on your hard drive outside of the default location, use the "Add new soundpools" function (available from the cog wheel drop-down menu in the "Loops" tab), and manually point the program to the location of those soundpools.
I hope this helps!