ddmyx/objects/oAudioManager/Step_0.gml

if bgm_fade_timer > 0
{
    if --bgm_fade_timer == 0 && bgm_next != noone
    {
        if audio_is_playing(bgm_current)
            audio_stop_sound(bgm_current);
        
        bgm_current = audio_play_sound(bgm_next, 0, true);
        audio_sound_gain(bgm_current, 0, 0);           // 从0开始
        audio_sound_gain(bgm_current, bgm_volume, 500); // 500ms淡入
        bgm_next = noone;
    }
}