In a FULL ALARM CONDITION do you have 12v between BELL (Neg - signal) & 12v (Pos - Hold Off) at the sounder connections inside your control panel?
That will tell you if it is the Sounder or the Panel is at fault, lets not worry about cable faults yet, try the basics.
Basically, fully set your alarm and go outside, let it set correctly, come back in, Entry timer starts, walk into any area that is NOT on your entry route, should then get an instant alarm. DON'T TURN IT OFF.
Open the control panel and with your multi meter do as I've indicated above.
If you have no meter or are not happy doing this, 240v Mains Electricity in the control panel, find an engineer who will be able to sort it out