We have enough to go on.
You are overloading the microphone input of the sound card. There is a small, sensitive amplifier before the volume control on the sound card, so large signals will overload it (the 0.5 thing) and there isn’t anything you can do.
The Mic-In of the sound card is expecting a tiny, wispy, delicate mono sound signal from a microphone. The output of the tape machine is a large, powerful, Stereo signal. The two don’t cross well. If you have a deskside PC, change to the Line-In (blue?) instead of the Mic-In (pink?) and all your problems are over.
If you have a laptop PC, chances are you have no stereo Line-In and you have to buy one. These have been reviewed and seem to work well. People like the UCA 202.
https://forum.audacityteam.org/t/sound-card-reviews/8375/1
Alternately, you can do as the system was designed and plug the microphone(s) directly into the sound card. Microphones produce a tiny, wispy, delicate mono signal, so the match should be very good.
Koz