What cable do I need to connect a microcassette recorder to my PC sound card?
It depends on the microcassette recorder. If it has a regular small headphone jack, then just buy a 3.5mm Male to Male patch cable. Plug that into your computers Line In and set Audacity to record from the Line In input (edit → preferences → audio i/o menu).
You’ll have to use the headphone volume knob to adjust the recording volume in Audacity (set Audacity’s input volume to full).