Comment 1 for bug 486164

I confirm, this is definitely a regression of the new sound preferences app as compared to what the volume control solution offered. While most desktop users will not need the power of the old mixer solution, the optional input->output routing is clearly a normal desktop audio need. So in my opinion this function should be added to the gnome-volume-control in future releases.

As a workaround you can run the pre-installed app "alsamixer" in a terminal. In the playback page navigate (with [left] and [right]) to the channel that corresponds to the input you want to monitor. This could be "Internal" or "Mic" or "Line-In", ..., depending on your audio device. If the channel is muted (it says "MM" under the fader), unmute it by pressing [m] on the keyboard. You can also adjust the monitor volume by pressing [up] and [down].

Regards, Alex