what works for me is to generate a sine wave (I used
wavelab in windows),
then copy and paste it until it is several hours long, then insert 10
seconds of silence at the beginning. Start rolling your sine wav recording
from another machine with a known good soundcard, and start your test
machine recording in the silent period. Its super easy to sync up to your
original wav by deleting all the silence at the beginning.
Along these lines RME provides a number of audio test files that you might
be interested in. One advantage of getting them from a known source is the
ability to do level matching throughout your system.
http://www.rme-audio.de/english/download/audtest.htm