If you choose specific devices, outputs and inputs from the REW selection boxes you have the option to always adjust volume settings yourself via your soundcard's mixer or to allow REW to set them to the last used values on startup. This is determined by the check boxes below the output and input selectors. Note that REW may not be able to control levels under some operating systems (e.g. Mac OS X) or for some soundcards, in those cases leave the Output Device and Input Device set to Default Device.
The Replay Buffer and Record Buffer controls set the size of the buffers used when accessing the soundcard. The default settings are 32k (meaning the buffer sizes are 32,768 pairs of audio samples). If you experience occasional glitches or interruptions in the signal generator output try increasing the replay buffer size, but note that there are other possible causes of this, such as interference from wireless cards. Similarly if the captured audio signals (as shown in the Scope graph) have occasional dropouts try increasing the record buffer. Using larger buffers will increase latency (delays when starting and stopping replay and recording) but should otherwise not be detrimental. If you are not experiencing any problems with audio input or output you may wish to reduce the buffer sizes to minimise latency.
Make sure that the Input Channel is set to the channel you are using to connect the SPL meter (or mic preamp output) - if Use Loopback as Timing Reference has been selected in the Analysis Preferences the other channel will be used a reference to eliminate time delays within the computer and soundcard, this requires a loopback connection on the reference channel. If the soundcard (or something else in the input chain) inverts the signal check the Invert box to have REW automatically correct the inversion. If the input has a DC offset check the High Pass box to have REW automatically apply a 2Hz high pass filter.
It is a good idea to measure the soundcard using the Calibrate... button in the Calibration section before making measurements to allow the response of the soundcard itself to be compensated for.
Before making measurements use Check Levels to make sure the replay and input levels are set correctly.