Ah thats should be simple.
Make a variable, call it "allsounds".
Then add to each event where you play a sound other than loop.
On play: set variable to 1.
Else: set variable to 0.
Put the play "loop" event all the way at the bottom of the event sheet.
Let it check: If variable allsounds = 1
Set volume to low.
If variable allsounds = 0
Set volume to high.
Because the event sheet works from top to bottom you can do it this way.
All sounds are checked playing and if they dont play.
Then for each sound the "Else" event is triggered.
So it will be set to either 1 or 0.
Reaching the bottom of your event list then it checks if that variable is 0 or 1.
I hope i explained it simple.