Assuming you’re running Windows 9x/Me/NT/2000/XP, and depending on your sound card, there’s often a way you can do this with no extra hardware required (i.e., you don’t have to wire your line-out port into your line-in port).
Go into the Windows mixer (double-clicking on the speaker icon in the system tray is the fastest way to open this up, or you can find it called Volume Control somewhere under Accessories in the Start … Programs menu), choose Options … Properties, select Adjust volume for … Recording instead of Playback, and see if there’s a selection for Stereo Mixer, or Stereo Mix, or Mixer, or Wave Out, or anything along those lines – something that might indicate your computer’s regular sound output. Some sound cards don’t have any of these options under the mixer’s Recording controls, but a lot of them do – my laptop’s “Conexant AC-Link Audio” device has Stereo Mixer, and my third desktop’s AC97 onboard sound device has Stereo Mix.
Anyway, so make sure that that, at least, has its box checked, and that Adjust volume for is set to Recording, and then click OK. Your mixer’s playback controls will disappear and be replaced by controls for the recording devices, with a Select checkbox under each device. Usually the recording device selected is the Microphone or Line In device. So you’ll want to check the box for Stereo Mixer or whatever your sound card calls it, and then fire up your favorite recording software (if you want to record to an ordinary .WAV file, you can use the built-in Windows program Sound Recorder, which is somewhere under Accessories all versions of Windows, I believe – Accessories … Entertainment in WinXP) and start recording, then start playing your flash animation or anything else you want to record.
Obviously, you should avoid causing error beeps or any other noises while the recording is going on, because this method will record every sound that your computer plays.
Good luck … hopefully that will do the job. If I’ve forgotten anything in the above description, or made any errors, I’m sure others will correct me. And hopefully the bold coding is all done properly as well… we shall see.