Sound not closing properly after closing the GUI

[Datseris]

julia> using MIDI, Mplay

julia> mplay(testmidi())

When listening to the piece, try to close the GUI exactly when a piano note is being played. You will see that the note sound is not terminated properly and instead a constant humming persists indefinitely.

This humming only goes away once restarting Julia.

I think this relates to the fact the the Mplay GUI doesn't close "properly" (also same as #4 ) .