Siren sounds weird when driven through NPN transistor
You need to drive the transistor "hard" into saturation. Your 100k resistor limits the base current to \$ I = \frac {V}{R} = \frac {5}{100k} = 50 \ \mu\text A \$. With a gain of, say, 100 the collector current will be 5 mA and this isn't enough to drive the siren. You can confirm this by measuring the voltage between the collector and emitter.

Try reducing the resistor to 1k and report back.