Your audio assets can be grouped together from the IDE so they can be loaded and unloaded into memory as required by your game.
You can create these audio groups from the Audio Groups window - which can be opened from The Tools Menu at the top of the IDE - and you can assign your audio assets to a specific group from the Sound Properties Window.
Each audio group has its own gain. The gain of all sounds in an audio group is multiplied by that audio group's gain. The default value is 1 but it can be changed using audio_group_set_gain and retrieved using audio_group_get_gain respectively.
NOTE All audio groups can be accessed in code through a constant that GameMaker defines for each of the groups. The default audio group is audiogroup_default. To get the name of the group as a string you can use audio_group_name.