Simplify ShowSound() in FileMonitor.cpp.

Now if more sound types are found, they just need to simply be added to
the unordered set.

- Also changed ShowSound() to IsSoundFile()
- Fixed IsSoundFile’s definition in FileMonitor.h. This whole time it
has been defined as a void method, when in reality it was a bool
function.

- Changed the FileMonitor’s string parameters to be constant references.

Source/Core/DiscIO/FileMonitor.h

@@ -11,10 +11,10 @@
 namespace FileMon
 {
 
-void ShowSound(std::string File);
-void ReadGC(std::string File);
-void CheckFile(std::string File, u64 Size);
-void FindFilename(u64 Offset);
+bool IsSoundFile(const std::string& filename);
+void ReadGC(const std::string& file);
+void CheckFile(const std::string& file, u64 size);
+void FindFilename(u64 offset);
 void Close();
 
 }