service/am: Unstub SetTransparentVolumeRate()

Like the other volume setter, this mainly just sets a data member within
the service, nothing too special.

src/core/hle/service/am/am.cpp

@@ -99,7 +99,7 @@ IAudioController::IAudioController() : ServiceFramework("IAudioController") {
         {1, &IAudioController::GetMainAppletExpectedMasterVolume, "GetMainAppletExpectedMasterVolume"},
         {2, &IAudioController::GetLibraryAppletExpectedMasterVolume, "GetLibraryAppletExpectedMasterVolume"},
         {3, nullptr, "ChangeMainAppletMasterVolume"},
-        {4, nullptr, "SetTransparentVolumeRate"},
+        {4, &IAudioController::SetTransparentAudioRate, "SetTransparentVolumeRate"},
     };
     // clang-format on
 
@@ -139,6 +139,20 @@ void IAudioController::GetLibraryAppletExpectedMasterVolume(Kernel::HLERequestCo
     rb.Push(library_applet_volume);
 }
 
+void IAudioController::SetTransparentAudioRate(Kernel::HLERequestContext& ctx) {
+    IPC::RequestParser rp{ctx};
+    const float transparent_volume_rate_tmp = rp.Pop<float>();
+
+    LOG_DEBUG(Service_AM, "called. transparent_volume_rate={}", transparent_volume_rate_tmp);
+
+    // Clamp volume range to 0-100%.
+    transparent_volume_rate =
+        std::clamp(transparent_volume_rate_tmp, min_allowed_volume, max_allowed_volume);
+
+    IPC::ResponseBuilder rb{ctx, 2};
+    rb.Push(RESULT_SUCCESS);
+}
+
 IDisplayController::IDisplayController() : ServiceFramework("IDisplayController") {
     // clang-format off
     static const FunctionInfo functions[] = {

src/core/hle/service/am/am.h

@@ -81,12 +81,14 @@ private:
     void SetExpectedMasterVolume(Kernel::HLERequestContext& ctx);
     void GetMainAppletExpectedMasterVolume(Kernel::HLERequestContext& ctx);
     void GetLibraryAppletExpectedMasterVolume(Kernel::HLERequestContext& ctx);
+    void SetTransparentAudioRate(Kernel::HLERequestContext& ctx);
 
     static constexpr float min_allowed_volume = 0.0f;
     static constexpr float max_allowed_volume = 1.0f;
 
     float main_applet_volume{0.25f};
     float library_applet_volume{max_allowed_volume};
+    float transparent_volume_rate{min_allowed_volume};
 };
 
 class IDisplayController final : public ServiceFramework<IDisplayController> {