Hi there, just thought i would give my 2cents about what i have determined. I setup an alarm console with just a remote and DS, and powerhorn. I set off the panic and the powerhorn does just as you say it goes for about six sounds and then pauses.
Next i tried it with arm home and set of the DS, and still same thing about 6 sounds.
Again I tried it with arm away and same thing.
Thought that my horns were bad also.
Then I tried with a powerflash module which is used to make almost any kind of alarm system flash the lights and sirens so that you have the X-10 features with a different company alarm system or just a homemade alarm system with out a brain unit. Anyway I set off the powerflash module by the contacts on the module and the powerhorn continued to sound as long as I kept the contacts closed.
So I guess what I am getting at is that the powerhorns seem to be fine, The problems we are having have to deal with the console. My console was purchased within the last year so it is probably a new design, I don't have an older console to try it out with to see if it does the same. Maybe the older ones do it also and no one has actually let it sound the sirens long enough to hear the pause.
Any guru's reading this that have been with X10 for many years, maybe you could put in some earplugs, and sound of the alarm and see if you in fact have a pause after about 6secs or so. Lights continue, and console speaker continues without the pause, as does the mini horn, the problem just seems to be with the powerhorn.