The diode and the light
There is a coil inside the buzzer, and a coil switched off keeps its current flowing for a moment. The SS14 across the buzzer gives that current a loop back to VCC, so the transistor's collector stops a few tenths of a volt above VCC. The red LED beside it lights while the transistor conducts: proof that SIGNAL arrived, not that the buzzer sounded.
A coil does not like to stop
The buzzer is electromagnetic: a coil inside the can moves the disc that makes the sound. A coil stores energy in its magnetic field while current flows, and when the current is cut it tries to keep that current going. If the current has nowhere to go, the voltage at the open end climbs until it finds somewhere.
Play it with the diode fitted, as the board ships. The transistor turns off, the coil's current turns round through the SS14 to VCC and back down through the buzzer, and dies away there. The collector never rises more than a few tenths of a volt above VCC.
Take the diode away and the same current has no path. How high the collector would go depends on the coil and the driver inside the can, and the buzzer's datasheet describes neither, so the figure does not put a number on it. The S8050 is rated 25 V from collector to emitter; the diode means nobody has to find out how close that spike comes.
Why a Schottky
The schematic carries a note beside the diode: it must be a Schottky. An SS14 conducts at a lower voltage than an ordinary silicon diode, under 0.55 V at a full amp and less at the few milliamps here, and it turns on quickly. Both keep the spike small.
The light
The LED and its 1 kΩ run from VCC to the collector, in parallel with the buzzer. While the transistor conducts, the collector sits near 0 V and the LED lights: about 3 mA with VCC on 5V, about 1.3 mA on 3V3, from a red LED's usual 2 V drop. Its current never passes through the buzzer.
That makes it a precise witness. Lit means SIGNAL is HIGH, VCC is present, GND is shared and the transistor is on. It does not mean the buzzer made a sound. Dark with SIGNAL HIGH means one of those four is missing, and lit with no sound means the buzzer is the problem.
When it does not work
No. The SS14 across the buzzer is it: cathode on VCC, anode on the transistor's collector, where the net list puts it. The schematic even carries a note that it must be a Schottky. Three wires to your board are the whole circuit.
The LED's current does not pass through the buzzer; it runs beside it. So SIGNAL, VCC and the transistor all work, and the fault is the buzzer or its two leads. Check the seal is not the only thing wrong: it quietens the buzzer, it should not silence it.
With VCC on 3V3 the LED's 1 kΩ has only about 1.3 V across it once the red LED has taken its 2 V, so it gets about 1.3 mA instead of about 3 mA on 5V. It is dimmer, not broken.
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