SPU: Connect 0x1F801DB8/0x1F801DBA writes

But I'm not aware of any games that actually use this, and if there is a
sweep active, it will be soon overwritten anyway.

src/core/spu.cpp

@@ -1096,6 +1096,22 @@ void SPU::WriteRegister(u32 offset, u16 value)
       DEBUG_LOG("SPU right external volume register <- 0x{:04X}", value);
       s_state.external_volume_right = value;
     }
+    break;
+
+    case 0x1F801DB8 - SPU_BASE:
+    {
+      DEBUG_LOG("SPU main left volume register <- 0x{:04X}", value);
+      GeneratePendingSamples();
+      s_state.main_volume_left.current_level = value;
+    }
+    break;
+
+    case 0x1F801DBA - SPU_BASE:
+    {
+      DEBUG_LOG("SPU main right volume register <- 0x{:04X}", value);
+      GeneratePendingSamples();
+      s_state.main_volume_right.current_level = value;
+    }
     break;
 
       // read-only registers