The bouncing sound

I"m finding that with

[Rectangle forward by 5]
[Rectangle bounce silence]

the rectangle eventually collides with the top of the world and bounces 
off in the opposite direction - with no sound ... the bounce is silent.

However, this script

[Rectangle forward by JoyStick's upDown * 8]
[Rectangle bounce silence]

doesn't allow the Rectangle bounce silently off the edge of the world. 
In this case, when the rectangle  reaches the top of the world, it makes 
a bouncing sound. The exact sound can be selected in the [Rectangle 
bounce silence] tile. But selecting 'silence' as the sound doesn't seem 
to work.

Am I missing something obvious? Shouldn't it be possible to silence the 
bounce?