I have created a global variable for my music that is an audio channel called BGM.
Then, whenever I want to change the volume I write something like:
Code: ags
and so on...
Then, whenever I want to change the volume I write something like:
BGM = aMySong.Play();
BGM.Volume = 100;
//code
BGM.Volume = 50;
// code
BGM.Volume = 100;
and so on...