Fix a few minor compiler warnings.

Make WAV play methods not pure virtual, so we can change them without affecting libretro.

src/common/SoundSDL2.cxx

@@ -72,7 +72,7 @@ SoundSDL2::~SoundSDL2()
 {
   ASSERT_MAIN_THREAD;
 
-  stopWav();
+  stopWav();  // NOLINT
   if(myWavDevice)
     SDL_CloseAudioDevice(myWavDevice);
 
@@ -409,7 +409,7 @@ bool SoundSDL2::playWav(const char* fileName, uInt32 position, uInt32 length)
   // - volume (requires callback using SDL_MixAudio)
   // - (un)mute
   SDL_AudioSpec wavSpec;
-  uInt32 wavLength;
+  uInt32 wavLength{0};
 
   // Stop any playing WAVs
   stopWav();

src/emucore/Sound.hxx

@@ -114,19 +114,20 @@ class Sound
 
       @return  True, if the WAV file can be played
     */
-    virtual bool playWav(const char* fileName, uInt32 position = 0, uInt32 length = 0) = 0;
+    virtual bool playWav(const char* fileName, uInt32 position = 0,
+                         uInt32 length = 0) { return false; }
 
     /**
       Stop any currently playing WAV file.
     */
-    virtual void stopWav() = 0;
+    virtual void stopWav() { }
 
     /**
       Get the size of the WAV file which remains to be played.
 
       @return  The remaining number of bytes
     */
-    virtual uInt32 wavSize() const = 0;
+    virtual uInt32 wavSize() const { return 0; }
 
   protected:
     /**

src/os/libretro/SoundLIBRETRO.hxx

@@ -151,10 +151,6 @@ class SoundLIBRETRO : public Sound
     bool toggleMute() override { return !myIsInitializedFlag; }
     string about() const override { return ""; }
 
-    bool playWav(const char*, uInt32, uInt32) override { return false; }
-    void stopWav() override { }
-    uInt32 wavSize() const override { return 0; }
-
   private:
     // Indicates if the sound device was successfully initialized
     bool myIsInitializedFlag{false};