Just plugging in a mono cable would not solve the problem, it still would only feed one channel in the mic input jack, which is stereo, and the other channel would be grounded. You need to be feeding both mic input channels with the identical signal, meaning you either need to use a stereo cable with stereo/mono adapter at the scanner, or a mono cable with mono/stereo adapter at the mic input. If you look at a stereo plug, there is a tip (left audio), followed by a ring (right audio) and then the barrel (common signal ground), whereas the mono plug only has the tip and barrel. When you plug a stereo plug into a mono jack you only get contact with the tip, and when you plug a mono plug into a stereo jack you get the same thing (only the left channel) except the right channel is shorted to the barrel.
What we are trying to do here is make sure we are feeding audio to both input channels so that we can rule out a hardware, firmware or software issue when encoding stereo with only left channel signal and then converting to mono downstream in software. Some sound cards have the ability to be configured for mono by specifying which channel to use, left or right, or "both", in order to obtain the mono signal. If you do not have those options, you need to bridge the inputs so that it does not make any difference, and if you do have those options you should only specify left or right (whichever has the signal, with a mono cable it would be the left) and NOT "both"... which would mix the channels (one live and the other dead) in software and possibly lead to the artifact we are hearing.