Rik and other readers, For those who want to calibrate the soundcard there is also a simple hardware solution : Take a CMOS4060 (14-stage divider with internal oscillator) and let it run with a xtal
Another solution is to run G3PLX's PSK31 SB software. There is a setup routine in the software that allows the Soundcard sampling rate to be calibrated. The procedure is given in the Help file, acces