Modifying a cheap PA Unit

[Patrol01]

Hey, one of our Auxiliary Police units ordered a cheap siren from a popular 3rd party website (That E one..) for their patrol car, and it works "fine", but the quality of it, well... Cheap, and it doesn't sound like the standard here, heck one of them sound like a car alarm....
Since they work so much with us, I decided to try to upgrade their stuff a bit, and I even found the "proper" sounds on our provider's website.

That system has two parts: The siren head, and the handheld controller unit with 7 buttons for each tones, and an extra button for the "PA system" for the microphone, but that's also bad quality.
They're connected with RJ9 or RJ11 (I can't really decide) the male end on the siren, and the female end on the controller itself.
The behavior of the buttons are pretty simple too: Press / Hold button -> Set tone plays through the "speaker".

Although I've seen the internal parts of the controller, it has an unnamed microcontroller on it so I don't really want to tamper with it. In the end I want to either create another similar circuit that is capable of playing my mp3s (multiple at once), or would a simple RJ9 (female) to 3.5mm (male) cable be enough for, like phone audio output?

How would you guys jump on that project?

Spoiler: Thumbnails

 

[ChrisP58]

How would you guys jump on that project?

Throw this away and buy a real one.

But, if you're really intent on modifying this, we'll need details of everything that is in the backshell of the horn.

 

[Patrol01]

I think buying the "real" one would be out of their budget.
I've returned the head unit since, and I'm not sure if I can get my hands on the horn itself, but I'll ask.

Thank you for the reply!