What are you looking to do? Remix them and try and get a better mix? If so then do as mentioned.If you're just looking to take a final mix and burn it to disc or make an mp3 you could get away with just going into the line in jack and use something free like reaper or audacity.
This would be using the built in soundcard which isn't the greatest for recording and the quality may suffer but it depends on your goals.I used to line in and mix down to the microsoft sound recorder just so i could burn my songs to disc.You're still better off though to get an interface.