quote: ...a Y cable that runs from two RCA jacks to a stereo 3.5mm plug which plugs into the laptop's microphone socket. It's obvious from the Audacity display that stereo is being received.

Could be. My laptop is 8 years old and I haven't looked at laptops since then. Maybe the mic inputs are stereo now. Are there stereo microphones to plug into them?

But note that if the mic input is a mono connector, you'd still see sound on both channels in Audacity. Both channels from the source would go into the 3.5 mm stereo male connector, but only one of those channels would be read by the 3.5 mm mono female connector on the computer, and that one channel would go into both channels of the built-in sound card.

If you're not sure, you could check by unplugging one of the RCA plugs from the Y-connector. If the mic input is stereo, you should lose one channel in Audacity. If the mic input is mono, you'd lose either both channels or neither channel in Audacity, depending on which RCA plug you disconnect.