Although MU 4.0 now has text palette entries for "mute" and "open", staff text won't allow a channel change.
3.6 required entering the mute type or open into staff text, then allowed affecting the voice of the trumpets in the properties. The "Change Channel" page allowed the change of voice.
4.0 now has to variations on staff text to somewhat simplify this, but I cannot find a way to "change the channel" and mute (or unmute) a trumpet. Was this access omitted?