Okay, so today I found out a glitch (on PSP), I'm not sure if this was already mentioned before but this glitch can change the siren of a possibly any 'siren-making' vehicle (I've tested this on a 'VCPD Cruiser' and a 'VCPD Wintergreen') with the use of a jumping-stunt that can possibly put the game into slow-motion for a while.
So this is how it works:
-Jump Onto a 'siren-making' vehicle
-Leave the siren off
-Find a ramp that can possibly put the game into slow-motion for a while when enough speed is put in
-Turn the siren on during the slow-motion
And now your siren on the vehicle will sound slower, deeper and different but when that siren changes... (e.g. you get out/off your vehicle and get back on, or you turn off your siren and back on) the siren will change back to normal.
[NOTE]: This also works when pushing down the the siren/ honking button to change the siren but if you let go it changes back
I have not tested this on any other GTA game and it'll be great if any can test this glitch on any other GTA game... Thanks!
Feel free to try this out on your own game!
EDIT:Also, if you pause the game and go back, the siren returns to normal
EDIT: This also works on the Ambulance Of VCS[PSP]
EDIT: Also on 'Police' of LCS [PSP]