Hi: No problem, depending on the system the Flashing LED's will either flash all the time that the box is powered i.e 24/7 to show that the box is a live box or will only flash when the system is set. As you said it could well be a problem with the Bell box itself, Unfortunately you could also have a panel problem.
As far as I know the sounder design has'nt changed so it should be a straightforward swap, if doing it yourself don't forget to remove all power from the panel before removing the sounder cover & use a pair of ear plugs when you connect the onboard battery in the sounder. The sounder can be quite loud even on a part charged battery & been known to startle people so badly that they've fallen off of ladders.
If i can give you any more help, please ask.
Andy @ Amman Security