Thanks Chris! Please note that latency (between start of play and record) should not be a problem with WinMLS. The latency must be known only in order to find the delay (or distance) from the loudspeaker to the microphone.
If you are interested in finding this time of flight, it is easily done using the loop-back feature (see Measurement->Sound Card Settings...). Sound cards that has a sample-accurate constant latency do also exist. We recommend and sell the Digigram and Lynx sound cards. Please see the Sound Cards section in the WinMLS documentation (or help file) for more details.