Audio Line-In on Ubuntu/Linux Mint

I am not sure which versions of Ubuntu this applies to but I know in 10.04 the loopback module to enable monitoring the line-in is not enabled by default. To enable this function issue this command in the terminal:

pactl load-module module-loopback

Your sound-card does it. Just open the volume control applet, and move "Line in" channel top the top (you might need to enable the channel configuration slide so it's not hidden).

Also try opening console (gnome terminal), and launch there:

$ alsamixer

You'll get a command-line volume control. Find "Line in" by pressing →, and hold ↑ untill it reaches 100%. Also check the slider's bottom: if "MM" is there then the channel is muted. Hit M key, and it will become enabled.

Cheers!