With regard the alarm the sounder it is like your house alarm, from what I have worked out to fix mine, if the main car battery voltage drops too low or is disconnected (say car not used very much and battery voltage falls over some weeks) the sounder thinks car is under attack and goes off ( just like your house alarm in a power cut if the main alarm unit back up battery is knackered, the sounder unit on the wall outside thinks it is being attacked as power to the main control unit is lost, which is why you hear house alarms going off during power cuts).
The sounder also has an integral battery which will likely be knacker at 10+ years old, a replacement meta unit is about £60 from memory and an easy fit.
The sounder also has an integral battery which will likely be knacker at 10+ years old, a replacement meta unit is about £60 from memory and an easy fit.