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 panel) 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 and 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 Left Channel as Calibration Reference has been selected the right channel will be automatically selected for measurement and the channel selection will be disabled.
It is a good idea to select Use Left Channel as Calibration Reference or to measure the soundcard using the button in the Calibration section before making measurements to allow the response of the soundcard itself to be compensated for. If using the soundcard left channel input as a calibration reference for the right channel signal the left channel signal must be looped back from output to input. Each measurement of the right channel is corrected to remove the frequency response variations found in the left channel. For this to be valid it is important to check that the left and right channels have near identical responses, if this is the case a loopback measurement of the right channel will give a flat response.
Before making measurements use Check Levels to make sure the replay and input levels are set correctly.