MOS SID sound card for ZX Spectrum edge connector (ZX-BUS).
 
PCB is compatible with SID 6581 and 8580, but also contains integrated SwinSID emulator.
SID accessible via #xxCF port, where xx - SID's register number.
You may connect your ZX Spectrum audio output to second 3.5mm jack and it will be mixed in sound card output signal.