audio: clear mixer routes on destruction

tests/Components/test_audio_system.cpp

@@ -218,6 +218,29 @@ TEST(AudioSystem, ListenerReverbMixerReceivesSend) {
     system.Shutdown();
 }
 
+TEST(AudioSystem, DestroyingMixerClearsDependentRoutes) {
+    AudioSystem& system = AudioSystem::Get();
+    system.Shutdown();
+
+    AudioSourceComponent source;
+    AudioListenerComponent listener;
+    auto parentMixer = std::make_unique<AudioMixer>();
+    auto childMixer = std::make_unique<AudioMixer>();
+
+    source.SetOutputMixer(parentMixer.get());
+    listener.SetReverb(parentMixer.get());
+    childMixer->SetOutputMixer(parentMixer.get());
+
+    parentMixer.reset();
+
+    EXPECT_EQ(source.GetOutputMixer(), nullptr);
+    EXPECT_EQ(listener.GetReverb(), nullptr);
+    EXPECT_EQ(system.GetListenerReverbMixer(), nullptr);
+    EXPECT_EQ(childMixer->GetOutputMixer(), nullptr);
+
+    system.Shutdown();
+}
+
 TEST(AudioSystem, ListenerComponentPublishesVelocityAndDopplerSettings) {
     AudioSystem& system = AudioSystem::Get();
     system.Shutdown();