Firstly, set up your audio. Most audio devices have a Line output sockets. Line level is a standard that specifies what voltage and current the audio device will send out. If I remember correctly it is 500mV for domestic and Semi Pro devices, and 750mV for Pro audio devices. I would guess that the standard set for most soundcards will be 500mA, but some of the newer Pro audio may be to the higher standard It shouldn't make too much difference unless you are recording at very high levels.
Turntables can be more of a problem, see below for more information.
You could capture audio from VCR's as well. Amplifier Tape out to Line in, Line out to Amplifier Tape in. This configuration is essentially replacing a traditional tape recorder connected to your HI-FI amplifier with your Linux system. The Line out to Tape in allows monitoring of the recording levels. Mike to Mike in.