Definitely try unplugging the siren itself. The thread that onehundred80 links to above give more info. The siren is under the plastic cowl below the windscreen, I couldn't get my wiper arms of but managed to get it out by bending the plastic cowl, don't blame me though if it breaks!. The alarm still functions with it disconnected but the siren won't sound of course. If you notice the lights still flashing occasionally after taking out the siren you problems elsewhere, likely under the boot floor. If you need some more info on how I fixed mine just ask. I guess that not all of them can be fixed but they can be replaced for £105.
Matt