"Isn't it possible to record the audio output of the computer?
Seems there isn't really anything to adjust.
There's not much to adjust, but... (and that's a big but) It's a sound-card issue. Some cards will allow recording directly off the output.. some won't. There really isn't much to do with the settings in SF itself except viewing the record level meters.
Generally, you need to access your soundcard's 'Record' options and select, "What you hear", 'Stereo mix" or something worded similarly.. This has been covered extensively before. Search this forum. and/or view this previous thread.