Make siren with UM3561 louder
Before you do anything else, you need to correct the error in the circuit that will destroy your UM3561.

The UM3561 is intended to operate on 3.3V.  

The circuit you reference has the UM3561 powered with 9V.  If you built it as shown, then you can expect the chip to die in short order.

Connect Vdd (pin 5 of the UM3561) to the junction of R3 and the zener diode.  That will provide 3.3V to the chip instead of 9V.  That's not an efficient way to do it, but it's more efficient than burning out chips.  You'll also need to connect the junction of S3 and S2 to the 3.3V source rather than the 9V.

With that out of the way, you have a few options to make the siren louder:


Use a better speaker.  Two speakers rated for the same impedance and power can have different efficiencies.  The more efficient one will be louder for the same power.
Use a speaker with lower impedance.  A 4 ohm speaker will consume more power for the same voltage than an 8 ohm speaker.  The 4 ohm speaker will (usually) be louder.
Use a smaller value resistor for R2.  More current through the base of Q1 means more current through the collector of Q1.  That means more current through the speaker, so it will be louder.
Higher supply voltage.  If you used 12V instead of 9V, then you'd have more current through the speaker and it would be louder.
Use a transistor with higher gain for Q1.  That will allow more current through the collector for the same base current.
Use something besides a 9V transistor battery for power.  The chances are very good that the battery voltage drops while the siren is sounding.




What I'd do:


Use 2 alkaline (or carbon zinc) 1.5V batteries in series for power.  C or D cells.
Get rid of R3 and the zener diode and power the UM3561 from the batteries directly.
Use a lower value resistor for R2.
Use a "beefier" transistor for Q1.  The BC547 is only rated for 100mA of collector current.  Use something that can handle at least a few hundred milliamperes.
Use a 4 ohm speaker instead of an 8 ohm speaker.


Points 1 and 2 save you the wasted energy in the zener "regulator" and provides you with a power source that can provide more current to the speaker.

The rest of the points let you get more current through the speaker despite the lower supply voltage.

Point 4 is needed because the BC547 would probably burn out if forced to deliver the needed current.



Here's the circuit implemented as I suggested:







simulate this circuit &ndash; Schematic created using CircuitLab

The BC548C has higher current gain than the transistor you were using.  I'm pretty sure it'll withstand the power dissipation.

R2 is lower so that you get more current through the base of the transistor, and the higher gain of the BC548C means you'll get more current through the speaker.