fix possible lock order problem

stepmania/src/RageSound.cpp

@@ -367,10 +367,14 @@ void RageSound::SoundIsFinishedPlaying()
 {
 	if( !m_bPlaying )
 		return;
-	m_Mutex.Lock();
 
+	/* Lock the mutex after calling UnregisterPlayingSound.  We must not make driver
+	 * calls with our mutex locked (driver mutex < sound mutex).  Nobody else will
+	 * see our sound as not playing until we set playing to false. */
 	m_iStoppedSourceFrame = (int) GetPositionSecondsInternal();
 
+	m_Mutex.Lock();
+
 //	LOG->Trace("set playing false for %p (SoundIsFinishedPlaying) (%s)", this, this->GetLoadedFilePath().c_str());
 	m_bPlaying = false;