Add the ability to duck volume when the game is inactive

osu.Game/Configuration/OsuConfigManager.cs

@@ -39,6 +39,8 @@ namespace osu.Game.Configuration
             };
 
             // Audio
+            Set(OsuSetting.VolumeInactive, 0.25, 0, 1, 0.01);
+
             Set(OsuSetting.MenuVoice, true);
             Set(OsuSetting.MenuMusic, true);
 
@@ -101,6 +103,7 @@ namespace osu.Game.Configuration
         MouseDisableButtons,
         MouseDisableWheel,
         AudioOffset,
+        VolumeInactive,
         MenuMusic,
         MenuVoice,
         CursorRotation,

osu.Game/OsuGame.cs

@@ -19,6 +19,7 @@ using OpenTK;
 using System.Linq;
 using System.Threading;
 using System.Threading.Tasks;
+using osu.Framework.Audio;
 using osu.Framework.Input.Bindings;
 using osu.Framework.Platform;
 using osu.Framework.Threading;
@@ -121,6 +122,8 @@ namespace osu.Game
             configRuleset = LocalConfig.GetBindable<int>(OsuSetting.Ruleset);
             Ruleset.Value = RulesetStore.GetRuleset(configRuleset.Value) ?? RulesetStore.AvailableRulesets.First();
             Ruleset.ValueChanged += r => configRuleset.Value = r.ID ?? 0;
+
+            LocalConfig.BindWith(OsuSetting.VolumeInactive, inactiveVolumeAdjust);
         }
 
         private ScheduledDelegate scoreLoad;
@@ -398,6 +401,21 @@ namespace osu.Game
             return false;
         }
 
+        private readonly BindableDouble inactiveVolumeAdjust = new BindableDouble();
+
+        protected override void OnDeactivated()
+        {
+            base.OnDeactivated();
+            Audio.AddAdjustment(AdjustableProperty.Volume, inactiveVolumeAdjust);
+        }
+
+        protected override void OnActivated()
+        {
+            base.OnActivated();
+            Audio.RemoveAdjustment(AdjustableProperty.Volume, inactiveVolumeAdjust);
+
+        }
+
         public bool OnReleased(GlobalAction action) => false;
 
         private Container mainContent;

osu.Game/Overlays/Settings/Sections/Audio/VolumeSettings.cs

@@ -4,6 +4,7 @@
 using osu.Framework.Allocation;
 using osu.Framework.Audio;
 using osu.Framework.Graphics;
+using osu.Game.Configuration;
 
 namespace osu.Game.Overlays.Settings.Sections.Audio
 {
@@ -12,11 +13,12 @@ namespace osu.Game.Overlays.Settings.Sections.Audio
         protected override string Header => "Volume";
 
         [BackgroundDependencyLoader]
-        private void load(AudioManager audio)
+        private void load(AudioManager audio, OsuConfigManager config)
         {
             Children = new Drawable[]
             {
                 new SettingsSlider<double> { LabelText = "Master", Bindable = audio.Volume, KeyboardStep = 0.1f },
+                new SettingsSlider<double> { LabelText = "Master (Window Inactive)", Bindable = config.GetBindable<double>(OsuSetting.VolumeInactive), KeyboardStep = 0.1f },
                 new SettingsSlider<double> { LabelText = "Effect", Bindable = audio.VolumeSample, KeyboardStep = 0.1f },
                 new SettingsSlider<double> { LabelText = "Music", Bindable = audio.VolumeTrack, KeyboardStep = 0.1f },
             };