I see you have the LCD back-light control and the piezo buzzer on the same port as the LCD interface and I suspect that's your problem. If the MikroC LCD routines aren't clobbering those two pins then you just need to take care not to clobber the LCD pins in your interrupt routine when you go to change either of those two pins. There's a chance you might not have to ditch the MikroC LCD library routines...
Are you using a 'shadow' variable for the back-light pin and the piezo speaker pin?
Are you using a 'shadow' variable for the back-light pin and the piezo speaker pin?
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