The only two things that come to mind are:
1. The siren is actually not as loud (not sure if it is the same part #) or is mounted in an area that muffles the sound a bit more.
2. The option I'd be more inclined to go with (easier to fix too), is your siren's battery is dead or about to die. I noticed on my 2009 Springer that the siren seemed appreciatively more pronounced when I had the battery replaced. When you arm, does it chirp 3 times instead of two? Not sure if this even matters, as I don't know if the siren pulls power from the primary battery only to rechard its own and uses its own battery to sound off, or if the siren battery is only backup for if someone pulls the primary battery.
Just thoughts and hopefully one leads to the answer, since I really have no idea otherwise.
Edit: Forgot that there could be one more reason, your siren itself is going bad. Try setting the alarm off vs just the arm/disarm blurps and see if it achieves decent volume.