create a global variable called musicVol and set it to 0 or whatever the volume is supposed to be ( goes from 0 decibels to -infinity I guess )
create another variable called fade = 0
have an event that sets music volume to the value of this global variable
Then on your command set fade variable to 1
create another event: if fade = 1 > every 0.1 seconds subtract 1 from musicVol variable. That would reduce music volume by 10 decibels per second, need it to fade faster set it to subtract 1 every 0.05 seconds or whatever fits your game.
I would also put some limits here. if musicVol<-100 set fade to 0 again so it stops fading because you can't hear anything at -100 anyway.
You can also bring it back up again, same way, just set fade = 2 and add event that if fade = 2 ads 1 to musicVol every 0.05 seconds, and if musicVol>than 0 and fade = 2 set fade back to 0 and musicVol back to 0, so it stops at 0 decibels