Yes, REW requires a proper line in signal from a proper sound card so the built in laptop audio will not work.
If the Xenyx had a USB interface, like the Alesis Multimix or Tascam US-144, you wouldn't need a separate soundcard. But since it doesn't you'll need a soundcard.But if I have a Xenyx 802/Behringer ECM 8000,so do I still need an external souncard??