I want to make some simple instruments that are controllable with both
midi and fltk widgets.

I hade some sucess with sliders being controllable with midi, but
having unexpected difficulty with buttons.

Is simply want to press a fltk button, and trigger an instrument. When
I uncheck the button, the instrument should end, just like when
pressing a midi keyboard note.

The manual page for FLbutton  seems to suggest that the instrument is
triggered when the button is checked but not when it is unchecked. I
get the behaviour that the instrument is triggered both when I check
and uncheck a button.

-- 
Joakim Verona