I wouldn't take any measurements until the soundcard had been calibrated and you have downloaded the proper meter calibration file. They will be quite inaccurate until you do so.Not sure how to calibrate sound card, but C weighting checked on REW, including C weighting SLOW on Radioshack digital SPL.
Read REW Help Files on calibrating the soundcard.
Download the meter calibration files from our download page.
Once that's complete and you've checked the soundcard calibration file for accuracy - take some measurements.