You'll have to put the midi's in the Music folder, edit the MAP info and Master Levels Library, recompile the library settings in the ACS folder (right click MasterLevelsLibrary.txt after editing it, go to scripts, and then click "Compile ACS")), and then replace the MSTRLVLL.o with the new file. There's a more detailed tutorial here: https://forum.zdoom.org/viewtopic.php?f=19&t=20795&start=60