Here's something you can try on Mac Sforzando to see if it works with the m4a files same as wma.
just put this in your RealTracks/Guitar, 12-String Acoustic, Strumming Ev 120 folder you should see AG1141.m4a etc..
right click Save Link/Target As
Guitar, 12-String Acoustic, Strumming Ev 120.sfz

Watch Video: https://www.dropbox.com/s/23mxfimdgf3ex62/RT-Strumming-SFZ-WMA-2.mp4?dl=0

Thanks for testing ! that seems to work fine on Mac.