I am using REW to measure noise levels and PSRR on power supplies and am very happy with the changes to allow mV uV inputs. I regularly use a +60dB LNA and that allows me to directly read the measurments.
I used a Rigol signal generator to input 1Vrms at 1kHz into my USB soundcard, a Focusrite Solo 3rd gen, no attenuation in the path. I do use BNC connectors and attached a Rigol bench multimeter to the output of the signal generator with a "T" BNC connector to ensure the voltage I input is correct.
With 1Vrms at 1kHz going into the soundcard, I hit he calibrate button and enter the reading on my multimeter, 997.7mV. You know have me wondering if I put the mV behind the entry..
Let me check that I entered it correctly and get back to you, it may be on my end.