Only left channel is recording
If your sound card is intel8x0 or maybe others then its alsa's fault. Its failing to set both capture channels to linein. One is left on the mic channel to fix this run this command.
amixer cset iface=MIXER,name='Capture Source' 4,4 numid=24,iface=MIXER,name='Capture Source'
Another fix is to upgrade alsa, but some people have said this problem still continues.
More information here.