don't load multiple copies of "common back" sound

stepmania/src/ScreenCaution.cpp

@@ -99,6 +99,6 @@ void ScreenCaution::MenuBack( PlayerNumber pn )
 		return;
 	this->ClearMessageQueue();
 	m_Back.StartTransitioning( SM_GoToPrevScreen );
-	SOUND->PlayOnce( THEME->GetPathToS("Common back") );
+	SCREENMAN->PlayBackSound();
 }
 

stepmania/src/ScreenCenterImage.cpp

@@ -126,7 +126,7 @@ void ScreenCenterImage::Input( const DeviceInput& DeviceI, const InputEventType
 	case SDLK_ESCAPE:
 		if(!IsTransitioning())
 		{
-			SOUND->PlayOnce( THEME->GetPathToS("Common back") );
+			SCREENMAN->PlayBackSound();
 			StartTransitioning( SM_GoToPrevScreen );		
 		}
 	case SDLK_RETURN:

stepmania/src/ScreenGameplay.cpp

@@ -1626,7 +1626,7 @@ void ScreenGameplay::Input( const DeviceInput& DeviceI, const InputEventType typ
 			LOG->Trace("Player %i went back", MenuI.player+1);
 
 			m_DancingState = STATE_OUTRO;
-			SOUND->PlayOnce( THEME->GetPathToS("Common back") );
+			SCREENMAN->PlayBackSound();
 			/* Hmm.  There are a bunch of subtly different ways we can
 			 * tween out: 
 			 *   1. Keep rendering the song, and keep it moving.  This might