MEP help says that combination of Ctrl + Shift + S opens MAGIX Soundtrack Maker. It doesn't work in MEP21 and didn't in MEP20. Does somebody knows anything about that?
Soundtrack Maker in MEP has nothing to do with Music Maker itself. However, Soundtrack Maker uses Soundpool loops the same as Music Maker except that MEP has template MMM files that can actually be opened in Music Maker.
You do not need and have never needed Music Maker to use Soundtrack Maker. What gave you the idea that Music Maker was needed? The fact that it was bundled with MEP?
Music Maker is free and a completely separate program; you can purchase add-ons individually or by getting Plus or Premium. BTW, the latest version of Music Maker Premium comes with a new version of Song Maker that is somewhat similar to Soundtrack Maker but is completely editable in Music Maker.