Попробуй вот так
Path.s="guitar_sample.mid"
If OpenWindow(0, 100, 100, 100, 100, "Beispiel: MP3 Abspielen", #PB_Window_SystemMenu | #PB_Window_MinimizeGadget | #PB_Window_MaximizeGadget)
ButtonGadget(11,20,20,20,20,"")
Buffer$=Space(128)
Repeat
EventID.l = WaitWindowEvent()
If EventID = #PB_Event_CloseWindow ; If the user has pressed on the close button
mciSendString_("CLOSE MIDI",0,0,0)
Quit = 1
EndIf
If EventID=#PB_Event_Gadget
If EventGadget()=11
mciSendString_("CLOSE MIDI",0,0,0)
mciSendString_("OPEN "+Path+" TYPE SEQUENCER ALIAS MIDI",@Buffer$,128,0)
mciSendString_("PLAY MIDI",0,0,0)
EndIf
EndIf
Until Quit = 1
EndIf
, а дальше методом тыка разберусь. 