audio_set_master_gain

With this function you can set the absolute value for the global volume of all sounds and music for a specific listener. The default listener index is 0, but you can use the function audio_get_listener_info() to get the different indices available for the target platform. The gain value is based on a linear scale from 0 (silent) to 1 (full volume) and will affect the relative volume of all sounds and music played from within your game through that listener.

Syntax:

audio_set_master_gain(listenerIndex, gain);

Returns:

N/A

Example:

var num = audio_get_listener_count();
for( var i = 0; i < num; i++;)
{
    var info = audio_get_listener_info(i);
    audio_set_master_gain(info[? "index"], 0.75);
    ds_map_destroy(info);
}

The above code loops through the available listeners and then sets their master gain to 0.75.