There are different versions of 'Music Maker' ... The basic version is free, which I suspect is included with Sound Forge AS 12. The Music Maker 2019 Plus is $60 and $100 (usd) for Music Maker 2019 Premium. See the comparison page.
I recall MM self-installed with the AS-12 beta test version I had, but I did not use it. Music Maker did not install with Sound Forge Pro 12, nor was there an option to do so. Contact MAGIX customer service for a definitive answer.1-305-722-5810 Monday to Friday 12 Noon - 4 PM CST