[Newbies] 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?