Device, scheme of which is published in the journal Popular electronics (USA), produces a positive pulse when the sound (for example, a shot the starting gun reaches the microphone B1 (see diagram).
Variable resistor R3 regulate the sensitivity of the device, and the led V4 indicates it is turned on. Open switch S1 associated with finish with a ribbon, breaks the supply circuit of SCR V5 as soon as the athlete crossed the finish line.
In your device, you can use any silicon transistors n-p-n and p-n-p conductivity, for example series KT315 and CT, SCR CW or with CO any letter index, led ALA, B.
As a microphone is recommended to use the "speaker" DEM-4m.