This is a built-in constant that represents the default audio group.
All newly added Sound Assets are added to this audio group by default, unless you create new ones and assign the sounds to one of those.
NOTE This audio group is a special audio group. Any non-streamed assets in it are guaranteed to be loaded at game start. Streamed sounds are also assigned to this group, but are ignored when it is loaded.
audiogroup_default
audio_group_set_gain(audiogroup_default, 0.5, 0);
The above code sets the gain of the default audio group to 0.5. The change in gain takes place instantly.