fix small bug in refactoring, that only shows in release mode

Author: Totto16

src/manager/music_manager.cpp

@@ -248,7 +248,7 @@ void MusicManager::hook_music_finished() {
 
     return static_cast<double>(result) / MIX_MAX_VOLUME;
 #else
-    return volume;
+    return m_volume;
 #endif
 }