Win 10 x64 Dell Inspiron desktop PC I have a cassette player connected to the PC via an RCA to Stereo Plug cord. I have been moving audio from cassettes to the system via Audacity with this. I thought I heard something strange during a recording and pulled the Right channel RCA plug from the player with no change in sound. I then put it back in and pulled the Left and all audio stopped. I put the Left plug into the Right jack and got audio, with the Right plug in either jack there is silence.I tested the cords by using the Stereo Plug in Headphones Out on the front of the case and moved the RCA plugs to Input and made a recording of a Left / Right test on Youtube on the Cassette deck. It recorded both channels with no problems.It looked like my Mic In was not seeing the Right channel and inputting 2 channel Mono to Audacity, as there had been 2 channel responses on the needles.[I earlier had a problem with Mic In not working at all and had warranty work done with a new motherboard put in, as that In jack is soldered onto it. I was afraid they messed up.]I then downloaded a VU Meter and looked again at the Left / Right Stereo test recording. With the Settings on Speakers/Headphones I get the same result as with Audacity: equal response in both meters for Left and Right signals. But, when I Set it to Microphone I see the expected Left and Right separation. And, when set for Microphone and I pull the Left RCA cord the sound stops but the VU Meter shows it still there. Replacing it and pulling the Right has audio out with expected VU response.

Stereo into the MIC IN shows up as Mono at the speakers and in Audacity. All music files etc, whether in my folders or online from Youtube, etc have correct Stereo Out.

Any help you can give on this would be appreciated, thank you.