If a sound is started, plays for a while, then stopped, repositioned and

started quickly, queued position data from the previous play will still
be flushed.  We clear already-flushed data in StopMixing (pos_map.clear())
so we don't have stale data, but some is being re-flushed afterwards because
we have the same ID.  Change sound IDs when stopping a sound.

(The symptom was the first frame in Play mode in the editor would process
using an old position; the position was usually later in the song, which
sometimes marked hold notes as dead.)

stepmania/src/RageSoundManager.cpp

@@ -122,14 +122,11 @@ void RageSoundManager::Update(float delta)
 }
 
 /* Register the given sound, and return a unique ID. */
-int RageSoundManager::RegisterSound( RageSound *p )
+void RageSoundManager::RegisterSound( RageSound *p )
 {
-	LockMut(g_SoundManMutex); /* lock for access to all_sounds and iID */
-
+	g_SoundManMutex.Lock(); /* lock for access to all_sounds */
 	all_sounds.insert( p );
-
-	static int iID = 0;
-	return ++iID;
+	g_SoundManMutex.Unlock(); /* finished with all_sounds */
 }
 
 void RageSoundManager::UnregisterSound( RageSound *p )
@@ -139,6 +136,14 @@ void RageSoundManager::UnregisterSound( RageSound *p )
 	g_SoundManMutex.Unlock(); /* finished with all_sounds */
 }
 
+/* Return a unique ID. */
+int RageSoundManager::GetUniqueID()
+{
+	LockMut(g_SoundManMutex); /* serialize iID */
+	static int iID = 0;
+	return ++iID;
+}
+
 void RageSoundManager::RegisterPlayingSound( RageSound *p )
 {
 	g_SoundManMutex.Lock(); /* lock for access to playing_sounds */