diff --git a/stepmania/src/RageSoundManager.cpp b/stepmania/src/RageSoundManager.cpp
index ca96485d41..88b2bfb3d2 100644
--- a/stepmania/src/RageSoundManager.cpp
+++ b/stepmania/src/RageSoundManager.cpp
@@ -161,7 +161,9 @@ void RageSoundManager::CommitPlayingPosition( int ID, int64_t frameno, int pos,
 
 RageSound *RageSoundManager::GetSoundByID( int ID )
 {
-	LockMut(g_SoundManMutex); /* lock for access to all_sounds */
+	/* You must lock this mutex before calling this function.  If you don't,
+	 * from any thread, the returned sound may be invalidated. */
+	ASSERT( g_SoundManMutex.IsLockedByThisThread() );
 
 	/* Find the sound with p.ID. */
 	set<RageSound *>::iterator it;
@@ -174,6 +176,10 @@ RageSound *RageSoundManager::GetSoundByID( int ID )
 /* This is only called by RageSoundManager::Update. */
 void RageSoundManager::FlushPosMapQueue()
 {
+	/* Lock, to make sure sounds returned by GetSoundByID remain valid until we're
+	 * done with them. */
+	LockMut(g_SoundManMutex);
+
 	queued_pos_map_t p;
 
 	/* We don't need to lock to access pos_map_queue. */