If I record music through Line In, I also want to be able to listen to it in real time while recording.
I don't want to use additional hardware splitters or software playthrough in Audacity (which has high latency). In Windows, I would normally enable playthrough for Line In through Control Panel or through the driver software for my soundcard. However, I couldn't find an easy way to do it through kmix. I eventually found that alsamixer will do what I want, and it's an easy enough fix.
Open a terminal, type "alsamixer".
Using the arrow keys, scroll right to "Line", hit "M" to unmute, and press Up until the volume is at its maximum. You should now hear all input through Line In immediately on your speakers.