Ubuntu Line in Playback Note working ?

Can't hear audio which is routed into your PC's line level inputs? That's because said inputs are muted by default in Ubuntu

To un-mute the line level inputs: Launch the Terminal (Applications-->Accessories-->Terminal), type alsamixer at the prompt, and then tap the Enter button on your PC's keyboard.

You'll see what you have to do to un-mute your PC's line level inputs (your keyboard's left & right arrow keys navigate, the up & down keys increase & decrease levels, and the 'M' key mutes & un-mutes the selected audio input(s) or outputs).