Your code is correct and it works correctly on a simple input. However, if your input includes a Replicator (or other meta-Sound that generates EventValues) the display-type definitions conflict. We have fixed this now, so you will be able to do it this way in the next software update.
In the meantime, you can work around it by changing the widget type for !Speed where it first appears (somewhere to the left of the Replicator-like Sound that is generating 16 of them). If you change the first appearance of !Speed before the Replicator to be a potentiometer, then the Replicator will make 16 copies of it, all as potentiometers.