The easiest way to calibrate the sample rate of a sound card

Most radio stations in Europe transmit a 19 KHz pilot tone which is pretty exact. Use a cheap mono radio receiver and spectrum analyser software like SpecLab to analyse how the sample frequency of your sound card differs from that.

a new form of DXing

If you're into abstract forms of DXing (listening to distant radio stations) like carrier DX (only receiving and analysing technical parameters of a TV or AM signal without listening to the programme), this might be something for you:
The pilot tone of radio stations varies from station to station by 1-2 Hz. This seems to be a criteria identifiying these stations. Some networks working with repeaters use the same pilot tone of the key transmitter on all repeating transmitters. But when the pilot tone is generated independently, one network can have varying pilot tone frequencies. -> experience