I have had this happen numerous times and have yet to figure out why. One method I have been testing that seems to work is after editing a mission and saving it, before trying to run it in either SP or MP mode, close DCS World completely and return to desktop. Then restart DCS World and run the mission. I will have to test this method more thoroughly to be sure if it is a solution but I would be interested to know if this method works for others that may be having the same issue. It seems that there is something about the save function that is not behaving normally.
Update:
I just loaded your V7 miz in the 1.5 ME and tested the sound files in the trigger menu and there was no sound. However when I opened the miz file and extracted the sound files they played just fine in Audacity. Weird!