This PCB uses an ATTiny85 microcontroller to play a digitised sound recording of a Sinclair Microdrive on a speaker, controlled by the LED output on either a vDriveZX or vDriveQL. The controller will start playing as soon as it gets power, which is controlled using an NPN transistor. The capacitor is there to make sure that any small power fluctuations don't cause the ATTiny85 to crash.
The sound playback code was originally written by David Johnson-Davies (www.technoblogy.com) in 2017 and modified to add the Microdrive sound data and set the correct playback speed.