Do you want the sound to stop when a MIAW is opened, or keep
Sound in the stage should actually keep playing on its own if
a MIAW is opened, therefore I am going to assume you want the sound
to stop playing (or at least mute)
Check out the help on whether or not the soundchannel is
busy. Use that type of script in the MIAW so it stops the sound if
running. Or you can similarly use the script in the stage to
monitor for MIAW's and pause or stop when one is detected.