I have this sample set and with a ViewSonic mine exhibits the exact same behavior. It seems the touch is only for momentary switching as the cymbal roll, crash, gong, triangle a few other sound effect work correctly. To get the Train whistle, Birds, Car horn, and other effects to sound properly, try mapping an unused piston or maybe a computer keyboard key as a work around.
On a traditional Wurlitzer theatre organs, these sound effect buttons were sequestered to a hidden swivel drawer (many times referred to as the junk drawer) mounted under the keydesk usually on the left side. You can create your own junk drawer by adding an under desk, pull out drawer with a Launchpad mini installed (7.28" X 7.28").
https://www.sweetwater.com/store/detail/LaunchP2mini--novation-launchpad-mini
I realize this is probably overkill for your request but my experience with HW is you can always make use of more assignable buttons.
Danny B.