With a pair of binaural mics placed 33.7 cm apart feeding into a stereo USB sound card (a Griffin iMic), I clapped near the left mic. It took 39 samples (884 µs) for the peak to travel between the microphones. That gave me a speed of sound of 381 m/s. A bit high, but the conditions this evening weren’t ideal, and I only set up the experiment with cursory care.
The iMic seems unusual in having a stereo line/mic input. Most other (cheaper) USB dongles only have a mono input.