Picking right Bipolar Junction Transistor
If you truly know that the load of the siren is 48mA at 12V it would be possible to switch this load with almost any general purpose NPN transistor. You could try a 2N3904. Tie the emitter to GND. Tie the collector to the negative lead of the siren. Tie the positive lead of the siren to +12V.

The base of the transistor will need a series resistor and you can try that with 1K ohm. The other end of the resistor would connect to the trigger device which will need to be either low near GND or active at the 2.4V that you mentioned.

Make sure that the GND of the trigger device is also connected to the GND of the siren and NPN transistor emitter. 

Selection criteria.

When picking a transistor for an application like this look for:


Saturated current rating. Transistor needs to be able to conduct the current that your load will require. If a general purpose transistor had a rating of say 200mA or even 500mA it would be just fine for a 48mA load.
Off voltage rating. When the transistor is off it has to be able to live with the voltage that is applied to it. A general purpose transistor with a 40 or 60 volt rating will be more than adequate for a 12V switching requirement.
Power rating. The transistor will have a maximum power rating while it is conducting current. The rating may be one in free air or specified with a certain type of heat sink which you will have to consider. For your application let's start with an estimate that the transistor can saturate to a 0.5V drop collector to emitter when it is on and conducting 50mA (close to 48ma but making calculation easier). The power of such situation is 0.5 * .050 = 0.025W = 25mW. A general purpose transistor with a free air rating of 250mW or better would be way more than needed.
Current gain. The transistor will require at least enough base current multiplied by the gain to achieve the collector load current and still be able to saturate the transistor. Base current can be forced higher and it will still work. If you provided a base current of 1mA and selected a transistor with a minimum gain of 50 then that would ensure that a collector current of 50mA would be possible. Any higher gain would work just fine for a switching application.