To record any sound coming out of your speakers using linux (Ex. music vids on mtv.com) you can use a nice little program called KRecord. I'm using a SB Live sound card in this example. You have to start alsamixer from a shell prompt with the command
Now look for the word "view:" at the top left. When you find it hit the tab key until the word next to "view:" changes to "All". From here use the right an left arrows to scroll back and forth between the bars. Find the one that says "Capture". Then hit the space bar to enable it. You will see a red "L" and "R" appear on either side of the bar. That means it's enabled. Then hit the up arrow key to change the level of the "Capture" input to about 40.
Now move to the next bar that says "AC97 Cap". Hit the up arrow until it's about 94. Move the "AC97" bar to 0. If you have a whine (feedback) then try to turn off other inputs (bars) that could be causing trouble. I had a problem with the "AC97" bar giving me feedback until I put it at 0. Bars with the names like "Master" or "PCM" are for your sound levels coming out of the speakers so keep them at a decent level. You may also need to try to turn on the "Mix" bar if something is not working. You can mute certain levels (bars) with the "m" key. The spacebar turns certain levels on and off. Do some test records with KRecord and play with the levels. If it your recording sounds bad try turning the levels down. Just hit the escape key when your done and run
This will save your settings you just set in alsamixer. You might want to mute or turn off your levels when your done. Just see how things sound.