Sound recording applications rely on proper settings within the audio mixer. With the mixer you can select the source for the recording (line-in or microphone) and the level of Gain for the recording. The mixer is also used to set output volume levels.
There are multiple possible sources for recording, including Line-In and Microphone. If the source is an external multitrack mixer or HiFi then plug it into the Line In and set Line as the Rec source. Note that both the line level and IGain need to be set appropriately, perhaps after some experimentation. Setting the gain too high results in clipping and setting it too low means that noise in the source can overwhelm the signal.
There are various tools to set the levels of various channels. Gnome Audio Mixer is available under Programs-->Multimedia, providing a graphical interface to mute/unmute channels and to select the channels for recording.
A command line alternative is the aumix command from the aumix package. You can query the current setting of the line-in and input-gain with:
$ aumix -lq -iq line 100, 100, R igain 30, 30
Here the line-in is selected for recording with 100% level. The gain is set at 30%. These work for my sound card.
Alternatively you can get all the current settings with:
$ aumix -q vol 62, 62, P pcm 63, 63 speaker 79, 79 line 100, 100, R mic 62, 62, P cd 100, 100, P igain 30, 30 line1 0, 0, P line2 0, 0, P phin 0, 0, P phout 0, 0 video 3, 3, P
$ aumix -l100 -i30 -lR
With aumix-gtk installed aumix will start up a graphical display that allows setting the values interactively.