Explicitly initialize `SoundTrack::volume`

While this is not needed because it's a static variable, it doesn't hurt
to be explicit, especially if it's going to be refactored in the future.

desktop_version/src/Music.cpp

@@ -246,7 +246,7 @@ end:
     static float volume;
 };
 FAudioSourceVoice** SoundTrack::voices = NULL;
-float SoundTrack::volume;
+float SoundTrack::volume = 0.0f;
 
 class MusicTrack
 {