To connect the electric guitar to the PC mic input, I built a mono-to-stereo adapter and it works: when running VLC (VideoLan) opening the audio capture, the sounds comes out of both channels, though in mono mode. The same when I run Audacity and playing back while recording.
But Rakarrack produces sound in left channel only. I even tried plugging the stereo mic that comes with the headset and Rakarrack does the same with the voice (left channel only), and yes the mic produces stereo sound in VLC and Audacity, so this is not a hardware or mono-to-stereo adapter problem.
Did you connect the mic input (mono source) to both inputs of Rakarrack? If you connect input to only one channel, then you will get only right or left (left default). If you are using qjackctl, you have to expand Rakarrack to see all the inputs. There is also the Aux input for the Vocoder.
Of course, you have to have both left and right channels connected to outputs left and right.
Just because you have a mono-to-stereo adapter doesn't mean you have the input properly connected to Rakarrack. If you depend on jack's defaults to make your connections, you can't know what will happen. You have to make connections on purpose.
Let me know if that helps. If not, them maybe there is something beyond the obvious. I don't have this problem, so I can only make guesses. My first guess is you haven't made the connections in jack correctly.